cortex, tiny, dingodile, or ripper roo
i think ripper roo and dingodile have the most variation of any bad guy, as cortex always was in giant robots ect
ripper roo - can plant TNT or Nitro, can jump on people in a variety of ways, and is just crazy so can do awesome things
dingodile has a flamethower that has so so so many options, and can use his tail as a spin attack
I'd like to see N.Brio in his hulk-like form from Crash 1, or The Evil Twins, but playing a bit like Pokemon Trainer from brawl, but having control of a few of the bad guys brains; they could even sit on their heads or shoulders as you played.
My favourite character other than crash in the entire series would have to be N.Gin but i wouldn't want him in this game. He'd suck in a fight. Cortex would be most likely but wouldn't have the most unique gameplay. Crunch, Ripper Roo and Dingodile would be really fun to play but wouldn't really stand a chance. Best case scenario for me would be Crunch. He would have a fairly good chance if they wanted 2 reps and he's like like Crash mixed with Barret from ff7. Who wouldn't want that.