Plan a level about the feeling you want to achieve :P
I have plans to make a level entitled "Momentum". Because during gameplay, sections of the game that let you keep your pace, while collecting bubble after bubble thus letting you get some huge rewarding chain feels awesome!!!! Just hearing the pitch of the notes get higher and higher is lust worthy @_@ I'll repeat stuff like that just to attempt to get it all in one run. I think it's rewarding, and totally challenging :)
Another section I really dig, are the ones where you use the trampolines to get higher and higher into the air. I would make a level that focused on something like that.
Essentially take elements that you thought were so much fun, and put together a level in a way you would LOVE to play it. Challenge yourself because you were so awesome at those segments, and the difficulty will come naturally :P
From 0 to KICKASS in .stupid seconds.
