I recently got a chance to play this episode, and it was quite enjoyable. While I did find a few flaws, I didn't see as many major problems like I do when I play most other featured episodes.
Anyway, my review:
First of all, while the design of each individual level was generally well done, the episode seemed more like a disjointed collection of really well made individual levels with the occasional cut-scene rather than a cohesive episode (
and I thought I was being original! Oh well). Not only is the difficulty scaling occasionally inconsistent, as Gameinsky pointed out, but also, each level has its own gimmick exclusive to said level instead of building upon the gimmicks introduced in previous levels to create a consistent theme throughout the episode. This is part of the reason level 15 is such a huge difficulty spike: it introduces a new gimmick (generated climbables) that the player has to get used to
while playing the final level (plus, said gimmick is quite difficult to control on its own, or maybe it was just my controller). Don't get me wrong: it's not a bad idea to introduce new gimmicks during the episode; just remember to continue building on the older gimmicks as well. Also, don't forget that, when introducing a new gimmick to the player, you should make sure that said gimmick is isolated so that the player can figure out what to do without being punished for messing up on something that he/she is unfamiliar with.
Moving on: in the intro cut-scene, you have a message appear telling the player that he/she needs to play as Mario to progress, and sure enough, the player is stuck if he/she decides to play as Toad. However, playing as Mario only makes the player hit a player-change block to change Mario into Toad! I know you had issues trying to get it to work with multiplayer, but how hard would it be to get the level to work if the player chooses Toad from the beginning? Just have the "Jump!" event swap the blocks under the player with Toad-only blocks so that if the player is already Toad, he/she can simply fall to where Bowser and the antagonist are instead of having to hit the player-change block. By the way, instead of having a message appear that says "Push up" when a cut-scene finishes, you can have the event force the player to push up by checking the "up" box under "player controls" (it's at the bottom-left of the event window). This will help your cut-scenes flow smoother.
In Peach's Castle, you have a Toad warn the player that, for the bonus levels, he/she will have to go though Peach's Castle again to replay a level if he/she dies, and that the checkpoints for those levels don't work. This is an obvious example of the "I couldn't get this to work right, so I'm warning you instead of fixing it" fallacy, and it's something I didn't expect coming from a well-respected level designer such as yourself. Rather than remove the checkpoints entirely, like bossedit8 suggested, it would be a better idea to check "Restart last level on death" on the wld file. Not only will this let the player restart those levels without having to go through Peach's Castle again, it will also get the checkpoints to work properly. Sure, this will cause Auto-start events not to work if the player dies, but you can get around this by having the player start in front of axes that trigger said events, as well as have said axes in front of the checkpoint if necessary.
Regarding Honey Hive Woods: was it intentional to have the Para-Goomba-sprite-swap bees turn into worms when they get stomped? Either way, that doesn't make any sense. I recommend having the worms sprite-swap another Goomba NPC and making the Goomba that's linked to the Para-Goomba that the bees sprite-swap uncollidable (jumphurt=1, nohurt=1, noblockcollision=1, speed=0) and look like the bee, but turned upside-down (so that it resembles normal death effect sprites).
In Piranha-Plant Creek, one of the clouds in the background is too high up and gets cutoff by the parallax. You can see it in the left of this screenshot:
By the way, what is the purpose of those two coins? It's very difficult to get them, and they don't lead anywhere.

Speaking of Piranha-Plant Creek, the SMB2 grass sprite-swaps blend in too much with the scenery, as can be inferred from Reign's post. I didn't even notice they were there until my second playthrough (I used a fire flower to get past the piranha plants), and even then, I had already made it to section 2, and I discovered them by complete accident. I recommend removing that sprite-swap and just using their default graphic instead; that way, they will stand out from the decoration and background. Also, you should add "grabtop=1" to the minecarts to make them easier for the player to pick up.
I only have one complaint about Dreadful Dungeon:
By having two Yoshi coins in such close proximity to each other, it makes it seem like you were running out of ideas and just stuck another Yoshi coin here. It would be better if you moved the Yoshi coin below the Thwomp to just under the second Thwomp in section 3.

Typo alert: the Mouser in the seaside town says "
your trying to stop my plans" when he should say "
you're trying to stop my plans." If you have trouble remembering which word is used in which instance, try reading the sentence out loud, but replace the word in question with "you are," and if the sentence still makes sense, use the contraction.
Regarding Water-Spout Sea: why are the berries in that level solid? Not only is that unintuitive, but
Darkonius64 already made a berry that works just like the ones in SMW. All you have to do to fix your berries is add playerblock=0, playerblocktop=0, jumphurt=1, and nohurt=1 to the berries' txt file to fix this issue.
Regarding Pirate Paradise: even though you apparently moved the fourth dragon coin up by one unit, that final section is still quite a huge difficulty spike (even when compared to the rest of the level) if the player wants to get all the dragon coins. At one point, I flat out missed it, so I just decided to finish the level normally without worrying about the dragon coins, yet the dragon coins' star was still revealed somehow. At first, I just thought that you forgot to hide its layer or to attach the last dragon coins to their layer, but that's not the case, and I'm not willing to go though the level selecting each possible event trigger to try to see what went wrong. Anyway, for the Lakitu's Shoe section, I recommend removing the invisible blocks, cropping the section boundary so that it's only one screen wide, putting everything in that section on a layer, and making said layer move to the left to simulate autoscroll (you can add extra events to make the dragon coins and the dragon coin star move as well).
By the way, if you want to have the water sprouts push the player upward rather than just act like boring old water, you can place a column of invisible sizables in front of it, with each sizable being exactly 1 pixel above/below each other (uncheck "Auto align" to accomplish this). Since the water sprouts themselves aren't too tall, you should be able to accomplish this without much (if any) lag.
Regarding Vicious Volcano, I agree with Gameinsky that you shouldn't have the secret star right there at the end, especially since you practically guide players right into the exit (even though that's not the level he was referring to). After all, you should
SMWC tips wrote:Try not to place secret exits at the very end of levels-- the player will basically have to go through the same level twice.
I at least recommend moving the coins at the bottom of the final two coin trails one unit away from the center so that they don't implicitly tell the player to move back to the center and so first-time players will have a better chance at being able to get the secret star.
Regarding the overworld map: while it is kinda neat that you have different branching paths for the levels even though the episode is linear, you should add shortcut warps back to Toad Town and/or Peach's Castle at the end of each branch; it's quite bothersome to have to backtrack aaaaallllllll the way back there after, say, beating Bowser's Fiery Fortress. Even if it doesn't make sense from a narrative perspective, it's still a good idea to have them since it removes forcing the player to backtrack through the world map (which is the most boring backtracking that can be done in SMBX).
Regarding Thwomp Caverns: The sign that lets players know that the red coins are for multiplayer should be placed before or beside the first red coin (in section 1) rather than in section 2.
Regarding Thorny Forest: I don't know who made those "thorn" graphics, but I stand by the fact that they don't look harmful. Plus, by having the vine aligned in the center of the sprite rather than the edge, the collision of the vines is slightly inaccurate and actually caused me to take an unfair hit at one point. Also, by having block-1 look like the p-switch, it's unintuitive that the p-switch
isn't what triggers its activation and deactivation, and that caused me to get killed on the last switch run. I recommend changing that graphic to a blue ! block so that the player will instinctively link them with the blue ? switch (the player will also know that the blocks won't disappear until after the p-switch runs out due to all the extra time left over after each p-switch run).
Regarding Malicous Molten Cave: while I liked the effect you had with the multiple Podobos coming out of the same unit of lava, the effect can be eliminated by simply off-screening the Podobos so that they all jump at the same time. You should be able to fix this by having an event that hides the extra Podobos when the player off-screens them.
Regarding Interstellar Adventure: Like I wrote before, I'm not a big fan of that generated climbable gimmick, as that gimmick was the cause of most of my deaths in that level. In fact, I had so much trouble with this level that I was disappointed by how easy the final level and boss were in comparison. However, I will say that my controller seems to have trouble with registering straight-up, and I had far less trouble with the gimmick when I replayed those sections using the keyboard, so maybe it's just me. However I can say for certain that their use in section 10 is rather cruel: the player is being chased by fast-moving lava, and since the generated climbables have the same palette as the stars in the background, chances are the player won't see the generated climbables in that section until it's too late.
Typo alert 2: At the end of level 15, Choofungus says "You may have
escape me this time, but it
is no matter!" when he should say "You may have
escaped me this time, but it
doesn't matter!"
Regarding Shadow Space Ruins: while looking through the level in the editor just now, I noticed that you placed player 2's start location really close to the checkpoint, meaning the player could simply start a 2-player game and skip the entire level except the boss. Speaking of the final boss, it was rather disappointing; it was just a Mother Brain sprite-swap, a couple Rinka generators, and some event lightning that didn't really add much to the fight. In fact, pretty much all of your boss fights were rather disappointing: the Bowser clones were literally just SMB3 Bowsers that the player has to shoot with a Billy Gun (a surprisingly common boss fight among featured episodes) and the "real Bowser" fight was just a Wart sprite-swap with a bomb generator, and both boss fights barely had any extra hazards in their arenas (and what
was there didn't make much difference). The only two boss fights that actually felt fresh and unique were the Mouser on a line-rider (I hadn't seen that before) and the Mouser in Seaside Town (that extra bomb-generator really threw off my timing).
By the way, while you did a pretty good job implementing the story, since the episode is playable with other characters, it would be a good idea not to refer to the player as "Mario" during dialogue. Toad got called "Mario" quite a few times during my playthrough.
Typo alert 3: During the epilogue, Rosalina says "little creatures
approuched me" when she should say "little creatures
approached me"
Regarding Desolate Desert: You should add a Mario/Luigi filter to that level since it's impossible as Toad. I made it all the way here:
and had to redo the entire level since Toad can't get past that part.
Regarding Mountainside Clifftops: I will give it props for having a clever hiding spot for the secret star (I felt that it was always obvious where the other secret stars were), but I found this to have the most problems of each individual level:
The coins for Toad are sprite-swapped to be wings, and I don't even think this graphic is used anywhere else in the level.

If the player falls down here, he/she won't be able to get back up, even with Life Alert.

I know those spaces are for the bomb generators, but the player can still skip a small part of the level by phasing through here. This can be fixed by moving the pipe on the right up by one unit.

This little effect takes too long to happen, and telling the player to stand still is pointless if the events already force the player to stand still. I recommend having the skull switch immediately reveal the explosion layers and move the player up.

Was this block supposed to trigger a leaf generator?

By the way, since the level has moving layers, the player will slide vertically when on a climbable tile (for me, it happened on the ladder in section 4, but not the vine in section 3 for some reason) so it is recommended not to use climbable tiles after the moving layer parts. Also, the level has a fairly pointless rainbow shell ride in section 5 (the only player input required is to jump off at the end), and this is especially out of place in the context of the episode since it's after Piranha-Plant Creek. Lastly, despite the fact that this level is fairly late in the episode, the level still feels the need to point out that collecting all the dragon coins reveals another star, and it decides to do this right when the player collects the last one, which is on a platform sinking into lava. Not only does this waste the player's time with something he/she already knows about, but, if anything, it will throw the player off and make the player more likely to get killed. That's a good example of something that would be good design for a stand-alone level, but not for a level in an episode (and even then, the player should be told that at the beginning of the level, not after collecting all the dragon coins).
One more thing: normally, I don't bring up music unless there are songs that aren't labeled with their origin, but during my playthrough, I encountered three custom songs that wouldn't play in SMBX:
DKC - Slammin' Steel
DKC - Jungle
Mario Galaxy - Final Boss
I don't know what the deal with those songs were, but I had to put them through
this music to MP3 converter to get them to work properly. I didn't even have to change their file names; simply converting the songs let them play properly. I assume that they worked for you because you use Wohlstand's SDL2 mixer, but they won't play in normal SMBX. (I don't use the SDL2 mixer since it can't play chain sound effects properly)
Speaking of your custom songs, where are these songs from? They're not labeled with their origin, and they don't have album credits:
Alien Theme
Black Horizons
Bowser Boss
Depths of Zarkhan
Final Battle
Shadow Space
Space Finale
In conclusion:
While there is plenty of room for improvement (like your bosses), this episode is still enjoyable by virtue of it being a collection of enjoyable levels. Keep up the good work!