Seriously, stop and think about that delightful possibility for a moment. Remember, rumors swirl that Cruise just can't bring in the audience butts anymore. (A story that was quickly denied even as the doubt seeped in by anonymous sources.) But Pegg? No worries! He was in Star Trek and is in tight with J.J. Abrams. Sure, he's not the headlining star but in a world where M: I 4 could be rebooted with a new lead, Pegg's role of Benji Dunn is rock solid.
You know what they should do? They should make Dunn the lead, or at least more of a c-ostar. Dunn could get a field promotion, and Hunt is his superior officer who has to train him up to face more action. Between Scotty and Benji Dunn, Pegg gets stuck in engine / lab rooms a lot. It's time to let him fire off a few rounds, stick some exploding gum to a wall, and tear off ridiculously realistic masks. I have no doubt Nicholas Angel could give Hunt a what for.