Like The Rock I think he is a guy with the skills, with the right role to be a big star. Shame The Rock generally picks poor films (and he is still doing very well out of things...)
In interviews the dude is a real character, he has plenty of charisma, now that may not translate to films, but I sense a role like this is ideal for him, and by all accounts this movie is HUGE (it could flop but with the rest of the cast being very stellar I doubt it).
I guess its finding the right balance. Dana generally demands 110% loyalty and at Rampage's end if somebody had said name 10 MMA Fighters who aren't that interested in the sport, Rampage's name would be up there.