I would say, even if it is a jam focus on design, that the game could benefit from different animations when the avatar is on the ground or floating because I thought for a moment that the avatar staying in the air was a bug as I was expecting a gameplay similar to VVVVVV. So a different animation/sprite when we are on the floor or floating could help the player to understand more intuitively that, even if it touch the ceiling he can not "jump back" and that staying in the air instead of continually going up is a feature and not a bug. As for the suggestions, I think it could be nice to add some challenges when you backtrack the level as I thought it was an interesting part of the game that could more exploited. Also, if you want to make more level, It could be interesting to add some "puzzle" level where you have to switch the gravity on and off to solve it !
I like your ideas! Different animations would not have been too hard to do. Mark also thought floating was a glitch lol!
The turning gravity on and off is very close to the core mechanic of a game that I'm working on right now. (You can play a prototype if you want. (It's called "The Gravity Machine"))
Thank you again for helping me to be a better designer!
(PS: no stealing "The Gravity Machine" main mechanic lol (I don't think you will tho lol (Do I say lol too much? LOL!)))