Make the background parallax and add some details and ti will look amazing!
I will try to make a neat background.
Slide a bit on the ice maybe if you don't already? Its sort of generic but meh. Maybe a little bit of snow particles when you run across snow? That'd be cool.
Also! Did you swim up the waterfall!?
Slippery ice is a given, maybe snow particles. I did not swim up that waterfall but there will be secrets like that in the final game.
Aw crap, he can shoot. So Cave Story then, not Montezuma's Revenge.