I'd rather they kept Tom Cruise in the role and if anything added , as Mixstar said, Brad Pitt as the bad guy. Aside all the fuss around Tom Cruise, Ethan Hunt and M I : 3 kicked ass.
Plus as someone noted, since Tom produced all the series so far, and was one of the main people who pushed for it to be made in the first place, I'm sure he's already got something which entitles him to the rights or at least involvement in any future movies.
Dino