I've had the same problem in the past. I still haven't finished my 2nd release.

This is what you do, and it works remarkably well for me:

Make a list of all possible events that you could use for the level. When you get to a point where you don't know what to add next, choose an event at random and think of how you can use that event creatively. When the level is done, go over it a second time and try the same method to add more events to each "area." A good level has LOTS of events (used creatively, of course). Shade Castle, Alien Chemistry, and any of chandie's levels are good examples of this. A great deal of gameplay in JJ2 is based off of what you might call "gimmicks." Remember in games like Donkey Kong Country where you'd have to bounce off of enemies to get somewhere, or shoot yourself out of a barrel at the right time. It's very much the same thing. Each level you make is like creating a new game, and you have to base the level off of trademark event/scenery combinations like the loop-de-loops in Sonic.

There. That should do it for you =)