So my thoughts so far.

1. I love the boss fight in this level. Usually, whenever I fight a boss in a game in the NSMB series, I love to wall jump, so this adds a bit of extra difficulty.
2. I love the opening area as it is, but generally, if you're trying to make a Mario Level, I think you should have the first area introduce the main mechanic and make it a safe zone for experimentation with the mechanic. Generally, you nailed the first part, but I think you should try to improve on the second area.
3. The star coins are meant to be a challenge to get, or at least hidden. In this level, the star coins are basically shown directly to you and not hard to get at all.
4. The second and third area of this level is way too bland, especially the second one. I think you should add some artifacts, make it more interesting, etc... This is also apparent on the water section in the third area.
5. For the third area, I've noticed that the time it can take to get the power up is too short and it'd be a close call. What you should do is have a little section off screen, where it'll take you right below that terrain, where you can get a star coin and that powerup. You can also hide it with Layer zero as well as the off screen thing I told you earlier.

Overall, I adored the level and this is a great concept. Just these few tweaks would be good