Make a low-res (320x240) saniman mod with WASD movement and orthogonal aiming. Give him evolutionary abilities based on a skill point tree. It would be like a multiplayer 2d dota + gang garrison.
edit:
elaborating on this idea, each player should be able to pick from 1 of 4 babby classes (saniboy, something, something, and maybe neneboy). as you level them up by getting kills or whatever, you put skill points into stuff and then you can unlock evolutions where your sprite changes.
^^ another class could be a moon rabbit babby