I've been building a (not very detailed as of yet) Jelly Arena for fighting in.
Basically the floor and walls (and later roof and randomly appearing obstacles) will bounce you off of them when you touch them, to varying degrees based on colour of the brick you touch.
Projectiles also bounce around off of the bricks too.
I plan to include some randomly appearing obstacles, like walls and walkways/pads in the air so that the game gets more variation.
Not sure what weapons or things to use in it as of the moment.
And I wish I knew how to event well so that I could make all the bricks randomly change colour and change how much they bounce you by.
It's pretty interesting as of the moment, but It's not complete and I haven't had anyone around to test how it's like to fight in it.
You bounce quite a lot (even on the weaker bouncing bricks, if you land on them with enough force) so it might need some tweaking.
If it goes well, and I continue with it, and some people enjoy it, I may look to having it hosted for some time over the holidays.