There's no way the Republicans would run him against Trump because it would split the vote and ensure a Democrat win. If he were to actually run, it would have to be Democrat or 3rd party.
Running Libertarian would be the best possible scenario for the party. Gary Johnson has backed out, so there's a vacancy, but he's still fighting the Debate Commission's bullshit standards (and winning). A big name like The Rock would bring much needed attention to get them on the debate stage, even with the 15% requirement.