i have good ones:
1. let the layer 8 (textured) background NOT USE layer 5 speeds but just layer 8
2.let the layer 8 (non-textured) background handle the speeds set (not that it only forces speed 0)
3. be able to choose when you darken the level(lightning stuff, event or properties) not making a lightly ring around jazz/spaz.
4.a one way that goes down! "if you come from above and you touch the ground you go trough it and then you are not able to jump up from beneath again"
5.more background layers
6. more foreground layers
7.maybe an extra sprite layer, which makes the layer 4 and the extra 1 both letting you walk on the masks.
8.let the springing shoes stuff work. that it actually MAKES you higher jump after picking this pickup (like in jj1)
9.maybe this also for a faster running etc.

i think most is impossible but some maybe can handled.
anyway, think about it guys.

** EDIT: 10. a spring event that doesn't fall down if set on air running the level.