IMO I believe if the story line allows us to have Sunstorm present in the movie. Starscream as we know it is the assumed leader of the 'Cons. I think a good story line would include Shockwave arriving to earth with Sunstorm and Skyfire who mutually have a strong dislike or hatred of Starscream. Skyfire would defect to the autobots while Sunstorm virtually annihilates Starscream in the swing of it all. I would put off resurrecting Megatron until the end of the movie. In the third movie Sunstorm would have seemingly became a loyalist of Megatron when in a pivotal moment of decepticon victory Sunstorm turns on his leader destroying him or almost doing so and a putting some other decepticon carcasses in the dirt leaving the autobots puzzled. He rejoins with a more powerful and merciless Starscream who seeks total domination inheriting the characteristics of Galvatron (on the verge of insanity and total rage) and enlisting the help of The Fallen. I personally think this could be a good storyline to follow of course some tweaking but why not.