I will start with the simple and impactful, and then dive further and further into the ridiciulously-high-effort and maybe-best-kept-for-a-future-level-using-this-mechanic tier stuff. Novel mechanics like this (it's basically Blue Yoshi but nobody ever uses Blue Yoshi so I'll call it novel) get the idea-gears turning rapidly, and the first run is always a bit of a prototype. Doesn't mean the idea should be ticked off as done-and-dusted afterwards :p
A1. If not remove, reduce timer by 50%. The level will not be significantly more difficult by letting the player ascend twice as high as they can on their current charge. This change is especially nice if #3 is impossible.
A2. Allow holding the button instead of mashing. This is mostly just to spare people's fingers.
A3. Change the 2nd screen so that players are forced to land before diving into a bottomless pit. This seriously got me on both of my runs, no joke.
A4. Give the player their regular jump when jumping from the ground first, then start flapping once the player's 0x11C offset has reached 0. Starting with a flap just feels kinda janky.
A5. GameData the reverse death event to auto-skip it on subsequent runs.
Cool, that's the easy stuff. Let's get into effortville.
B1. Embrace a more open structure. This gameplay mechanic does best in open areas, but all sections are a screen tall or a screen wide. You saw me try to go the wrong way in many places in the video, because I thought it'd be rad if I went there and there was something there.
An easy way to add openness to the level without impeding on existing design would be to have a suspicious hole in the ceiling of a horizontal section that leads to an optional area. What would the reward be? That's always the tricky thing, isn't it... It doesn't have to be tangibly useful in the level, it can just be an acknowledgement of the player's cleverness. A 1-up, an NPC, or a scenic shot of the landscape would do just fine.
I keep suggesting optional areas in these reviews and it really makes me disappointed that the SMB3 Chest NPC didn't win in the head-to-head for Beta 5. Dang!
B2. If you can come up with a better obstacle type to weave into the level (like a bramble maze for instance), I would suggest removing every red arrow, at least in the autoscroller. That's just functionally equivalent to an instakill block, so why even give mushrooms if those arrows will kill you anyway, right?
B3. More obstacle types in general. I'm not a fan of this "use as few enemies as possible" type design. SMBX comes with like 600 enemies out of the box, and I can't believe none of these would be interesting to dodge in this mode. WARNING: This point is HEAVILY biased toward what I like. Take it with a TABLESPOON OF SALT.
And the stupid and unworkable, the "save it for the sequel" stuff:
C1. Metroidvania
This is a continuation of B1, but if you stretch the open areas out to their natural extreme, you literally get the videogame Owlboy. It actually works pretty well to just dump the player in an open area, show them a locked gate with 2 un-lit towers beside, and give them cables leading left and right and tell them to figure it out. They can fly all over the place, still constrained by the timer, and they just need to figure out how to get to the endpoint of the cable by weaving around a nonlinear area. It's a kind of emergent fun that's sadly absent in more room-by-room level structures, but one which I can see this mechanic, especially with the timer intact, excel at. Something for the sequel, perhaps?
C2. Blue Yoshi.
Near the start of the review I kept referring to this owl as Blue Yoshi. They are pretty similar in how they work, aren't they? I guess you don't get the benefits of Yoshi's tongue. But maybe having that would allow for a more interesting design? This is far outside the scope of this level, especially with the deadline encroaching so soon, but you caaaan get an auto-infinite-flight Blue Yoshi that cannot be dismounted and will not flee when hurt with maybe a dozen lines of lua, by manipulating the player's memory in certain ways (setting mount, cancelling onPlayerHarm, disabling spinjump, forcing yoshi wings active). If you want to make a level like this in the future that also features a "weapon" in the form of Yoshi's tongue, consider this an easy cheat code to get that, lol.