I would make a simple sprite-based evolution game. You collect sprites which are arms, legs, heads, main bodies and such. The player would sometimes need to collect sets of legs & arms to make some combinations work. Each "body part" would have its own capabilities (maxium rotational arc, reach, damage, special damage, health buffs etc.). Then you could go around in some boss/pvp battle environment and fight with this creature. Movement controlled by arrow keys and then you can map special arm/leg special attacks to mouse and keys. Every time you win you can pick one body-part to keep. For graphics - look at how plants vs zombies was made. They have a modular system for arms/heads/legs etc. The biggest challange would be to figure out how animations would work if you have 1,2,3 or 4 legged creatures. But perhaps you could base the whole thing on some templates. I would also make a marketplace where players could exchange body parts, but with a real cash fee attached to it so that I can get rich.
So, basically EVO but with a marketplace system and a micropayment system? EVO was a cool game, but from the art side of things I think it's actually easier to do the mix-n-match bodyparts thing in 3D, especially since it also makes custom colors/texturing easier. Doesn't have to be as complicated as Spore's system to look and work nicely.