I have been doing research on whethter Unicron will appear in this movie series. I believe that the Matrix of Leadership's apparent appearance in the Transformers movie reinforces my theory.
Recently someone mentioned the fact that the Matrix of Leadership may not exist in Michael Bay's Transformer series, I hope that will prove to be false.
If the Matrix is indeed in the movie it contributes to the possibility of Unicron being in the third movie.
If you watch Transformers Optimus Prime opens his chest at one point revealing what may just be his Spark, or it could be the Matrix, as I think it is.
Michael Bay has said that Unicron would be fun to include in the next movie for "Comparison sake"
Here are some other reasons that Unicron may be in one of the next movies:
Both the Fallen AND Sideways have been confirmed to be in the new movie.
The Fallen is a servant of Unicron.
Sideways is often depicted as a herald of Unicron. In the movie he turns into an Audi R8
Megatron is brought back to life in this movie, could Unicron in the form of Sideways be involved in Megatron's resurection.
It is confirmed that Megatron's fallen troops will be resurected as well.
If the Matrix is present in this continuity could this mean Prime will pass it on to a successor.
It is rumored Prime wil die in this movie or the next.
All of these facts fall into all interpretations of Unicron's storylines, so it is likely that he will be in the next two movies.
Recent spoilers have revealed that the Decepticons are looking for a powerful weapon in Egypt. It is hinted that the weapon will be used to destroy the Sun, or it could be used as it was in the Unicron Trilogy. In the Unicron Trilogy the Decpticons weapon was used to help bring Unicron to life.
These are but a few of the many reasons that Unicron will appear in a future movie