Details of the Transformers 2 game have been released online, and come from the people over at IESB
“Make every second count Transformers 2: The Ultimate Battle, as you choose to seek revenge as the Decepticons or defend Earth as the Autobots. Go beyond the movie and drop into pressure-packed missions where seconds can make the difference between survival and destruction. Play as a variety of Transformers, such as the evil Starscream or heroic Optimus Prime, to either demolish or protect fully destructible environments around the globe. A new Transformation system gives you more detailed control as you instantly switch between Robot, Weapon, and Vehicle modes at any time to drive, fly, fight and shoot your way through gripping levels. Then make your choice online as you go head to head against friends in multiplayer battle zones. Your alliance will decide whether Earth’s greatest cities stand or fall in ruin as you experience the ultimate level of intensity in this race against time.”
So, according to this, the ultimate villain in the ultimate battle is Starscream. We see him flying away from Earth at the end of the first film presumably headed to gather a new team of Decepticons to take on the Autobots.