I'm pretty sure alot of people would agree with this idea, why doesn't naughty dog just simply allow us to play as villains in co-op arena? You can choose hero/villain in free for all and three team deathmatch. Some people just play co-op and never touch the competitive side of the multiplayer and so never get a chance to play as villains. With the new co-op adventure DLC out, it seems even more possible to do so.
I want this sooooo badly. Co-op is my solace when I'm getting punched through my rolls by M9 ROF scrubs and campers at Chateau, and I really like playing as a ninja. I'm hoping that this happens at least with the rumored "shades mode".