An upcoming Dark Souls mod from DaTeHaCKs will let you take control of the game's bosses. It's called the Dark Souls Boss Arena Mod and it's slated for a September 16th, 2015 release date.
The trailer features the player controlling Manus, the Asylum Demon, Ornstein, Smough, Quelaag, and Kalameet, but according to the developers, you'll be able to play as all of the bosses from the game and every enemy character as well.