I think you can add some power up.
One random power up in a box per level.
Freeze - make the enemies freeze a few sec
Generator - energy doesn't decrease a few sec
Armor - invincible for one hit from enemies
Lightning - Everything go slower except human a few sec
Ground Slam - All enemies in radius fall except Spidertron
....
and you can bring them to another level but cant have the same..
any idea? just give it
OK?