If there will be a TF4 then Unicron will probably be one of the villains they'd consider. The problem is that Unicron as the planet-sized Transformer would be impracticle in the Movieverse. So how would you fit the Planet Destroyer into the Movies? They could go the Prime route and have Unicron basically be Earth. For me however the best way to include him would make him pretty freakin huge but not planet sized. Maybe the size of Mt Everest. Perhaps they can even put him responsible for the KT Event when he landed on Earth.