It seems that time is repeating itself, or atleast for one member of the Autobots that is, Optimus Prime. Optimus Prime who is the leader of the Autobots has normally been gifted with the ability to combine with his trailer (Optimus combined with his Trailer in Armada). However this is the first time in live-action he is going to do so. How i found out you ask, some recent concept art by Josh Nizzi, ofcourse (Below) The concept art shows that the trailer can seemingly Transform into different segments to form added weapons. Including an Energon Axe, a stabbing weapon and a large gun and shield.
Personally i dont like the large sheild and gun shown in concept 4 as i would rather want Optimus to gain more weapons rather than hide behind a shield.
Well anyway you heard it here first, Optimus Prime will combine with his trailer, aslong as Bay uses Josh Nizzi`s concept.
Update: It seems this concept design is from TF: ROTF, however, it is still cool and could still possibly be used for TF3.
Update 2: New image found relating to the combined mode (Shown above)