I wouldn't mind Joe Caranhan (Narc, Smokin' Aces) directing this. He was once attached to direct Mission: Impossible III but then dropped out. Of course, his next project is The A-Team so that might prohibit him from directing another blockbuster so soon.
Joe Carnahan won't be coming back. He had his own Mission: Impossible story which supposedly was going to be a serious R-rated spy movie. He's not going to return as Abrams' hired hand, especially since he's made the claim that his MI3 would've been so much better than what Abrams made.
If the script being hammered out deals with AI, maybe Oliver Stone would be interested? He was mulling over doing an AI-themed Mission: Impossible II.
I wonder what David Fincher's Mission: Impossible III was going to be like.