There is no word on wether Shia LaBeouf will return or not. The studio might just start fresh and create some new lead characters. As long as Josh Duhamel returns as the Transformers amry friend, then I'm cool with that.
If Bay doesn't return as director, I wonder who will take over? I think it should either be Rob Marshall, Zach Snyder, Darren Aronofsky or even Spielberg himself, rather than him being only the producer.
