this topic is a very hard to invent a game for jam.

Some inspiration and ideas for you:

The Avengers - a group of misfit superheroes who come together to fight the battles others can't.  

Justice League - same idea 

A game focused on team play, maybe a game where you play as two characters (one acts as a platform that can transport the other player to progress for example.)

Hope these give you some inspiration!

It really is hard... especially when you wanted another theme :/

You can start planning for the project and do some art.

Was i the only one, who just got an idea about the theme the moment i saw it? even though i wanted another theme

I also got an idea almost instantly