Will Arnett's voice already sounds like a cartoon parody of tough guy actors, so it makes sense that he'd get cast in Johnathan Liebesman's upcoming CG/live-action hybrid Teenage Mutant Ninja Turtles film.
But who is he going to play? We don't know yet. They're keeping it secret. Because this is a film that demands secrecy. He could provide a suitably cheesy Casey Jones or maybe even a silly Shredder or Splinter, though. If it's a mo-cap performance there's really no limit to which character he could take on, save maybe for the nerdy Baxter Stockman. So he's probably playing Baxter Stockman.
Why do I keep covering this film? I really have no idea. They just keep adding strange elements to it. By the time they're done casting, Jonathan Winters will be playing Krang and Temuera Morrison will be portraying all the Foot Soldiers.