Challenges are almost like having different modes. You could make a race type challenge or something like that. There could be more to challenges than you think.

What ideas do you guys have?

I'd like to see a simple survival challenge mode... start the player (and his team) in a room with a few defensible positions, and send in ever-increasing numbers of opponents to see how long the team lasts.

That sounds like Tower Defense to me but endless.

I think TD could be way more interesting/fun with different maps. Also you need a filter if you want a time limit or a survival mode.