There are two main ways that you can cast a big action adventure movie. You can go the anonymous route, the way that Die Hard did. Bruce Willis was only a minor celebrity at the time, and the choice to make him the lead of an adrenaline-soaked thriller was a conscious decision to keep things grounded and in the real world.
The second choice is to go after stars. This was the case in The Matrix, when Keanu Reeves was selected to play Neo. It also happens in Wild Wild West, Independence Day or basically any movie starring Arnold Schwarzenneger. This ensures that audiences don't get sucked in too far, always remembering that the story they're seeing is a movie.
Director Michael Bay is apparently thinking about shifting from one strategy to another, according to IndieWire. The first three Transformers movies featured Shia LaBeouf as their protagonist. As a young up-and-coming actor, he didn't overshadow the spectacle of the franchise. However, with LaBeouf out of the series, Bay needs a new anchor. He's shooting for the stars in Mark Wahlberg.
At present, the two are working with The Rock on an action comedy called Pain and Gain, about a pair of steroid-abusing criminals in Miami. Wahlberg and Bay have dismissed the Transformers rumors, but there have been no flat out denials. Stay tuned to find out whether or not Marky Mark will end up starring with Optimus Prime.