In a Q&A at TFW2005, Transformers Revenge Of The Fallen writer Roberto Orci has provided some more answers, this time confirming that there will be no triple changers (ala Blitzwing) but there will be combiners:

Talked about it. Does a combiner count? We figure a triple changer is a good idea for part 3.

Also answered was the likelihood that Megatron and Starscream will be up to their old ways again, withbickering left right and center and that it has been discussed (no confirmation) in the writters room about the possiblity of Optimus Prime combining with another robot or even his trailer.

And for anyone wondering what Soundwave will trasnform into, nonbody has guessed it yet.