A little update for my project for those that are interested.
I've also done work on the second and third missions,
and have also started the bonus level for the first mission.

The first room of the bonus level for mission 1. I might change it a bit because I feel it's a little boring and repetitive.

Because Rex jumping is fun.

Ho boy. I died. I might tweak with this one a little bit as well.

More of the second world.

You can tell you're doing well when a shell kills you.

Another puzzling level involving the switch blocks.

The third world is underground. Here, Mario has encountered a subterranean lake.

This was taken only a second before I fell in the lava.

The intermission level of the third mission.You have to hit the Transport using the shell. The Transport are just ? blocks in disguise. I'm gonna use a Photoshop filter on them to help them blend in. However, there is a slight glitch, where when they are hit, only half of them remains, as seen above. Also, one of the blocks was flickering and disappeared when hit. I'm gonna have to play with that for a little while.
This is the world plan I have in mind
World 1: Grass
World 2: Desert
World 3: Cave
World 4: Snow
World 5: Forest
World 6: Beach
World 7: City
World 8: Clouds
World 9: Space
World 10: Bowser's Hideout
If anybody has any ideas, thoughts or opinions on what I've done, don't be afraid to have your voice heard - well, really, your comment seen.
Any ideas for levels will be considered as well. I can't create 171 levels myself (120 main levels, 50 bonus levels, 10 hubs, 1 final boss)
Feel free to suggest any ideas for levels. They have to be the default level size (1 screen), but they could have scrolling layers to make the level seem more.