The thing about Hollywood is that just about any guy can be beefed up. You just have to put some trainers on him and put him on a killer diet. A lot of guys have transformed themselves for parts.
That said, other alreafy big guys I adore: Vin Diesel! (but he and Johnson just worked together F5 and it was awesome), Derek Mears, Tyler Mane, and Kane Hodder are cool to have around.
And Michael C Duncan is always a welcome sight in a Michael Bay film.