I would say a random fatality generator. For the 2D MK games, have a function that has access to all of the current character's frames of animation, all props and helpers from the game, and have tons of preset actions for the opponents.

Making up my own fatality could be kind of funny, but creating something that generated a new F'ed up random fatality, or WTFatality as I like to call it, would be really fun. Just imagine what could be done with Sindel's friendship animation + Cyrax's bomb + opponent looking other way stuck in an awkward getting-hit animation. Priceless.

shang tsung swithching into sub-zero, freezes the body, swithesinto scorpion and does spear slices the head off , turns into normal and roundhouses the head to the frozen body and explode upper half, switch to smoke (robot) and lauch a bunch of bombs, after the explosion ice shards fall down, switches to reptile and acid spit evry remaining shard before it hits the ground.