I happened to stumble across this gem earlier today.
It's a game called Duel Toys and it's a crossover 2D beat em up with a HUGE roster of characters!
The fighting system is no slouch eitheir, it's actually got a good amount of depth to it. No character plays the same since they all have their own set of moves.
There's a counter system, recovery system, and the combos you can pull off are immensely satisfying.
Here's a gameplay video to give you an idea, it's jaw dropping.
And it's free
So for those who always wanted to pit Akuma against Sub Zero, go nuts
Character Moves List.