Inside a tank. Where Jazz sabotages stuff. And the wheels turn, along with the background moving, so it looks like the tank is actually rumbling along the ground as Jazz blows it out from the inside. And you could use the background from 7th Lava Fall, except with all of the buildings with large holes in them, like they've been shot by a tank.

In The Background. Mask parts of a background, and turn it into a tileset! EEEE!

A giant metal airship. I know this has been done before, but it was lost when J2C died, so another one would be cool!

Another sky tileset: Planes. You start in the cockpit of a plane, and you have to jump from plane to plane and kill all of the baddies in all of the other planes. The planes could be of any type, like biplanes, triplanes, big passenger planes, stealth bombers, blimps (possibly), etc, etc.

I'm out of ideas.