Autbots and Decepticons will all have modified parts.
Cybertron is being rebuilt by the remaining decepticons.
Megatron is revived and is called Galvatron.
A group of decepticons stumble across Unicron.
Unicron threatens to destroy all planets (Earth, Cybertron).
Sam and Carly's son, (At the age of 20), visits his grandparents. (Spike Witwicky)
He finds out that Judy is dead.
Ron lives alone.
He explains how Sam and Carly are living their lives somewhere else.
Sam told the autobots to stick with Spike before the events of the fourth film.
Leo returns, and is very successful, rich, famous, etc.
It takes place in the early 2030's.
Half of the movie is on Cybertron.
Part 4 and Part 5 will be a 2 parter, with part 4 ending with a cliffhanger.
Autobots and Decepticons ALL have more similar vehicles to their original G1 concepts. (EX: Insted of Black Ironhide, make him red, but he's already dead, that's an example.)
That is what I think, and hope, for to happen in TF4. Feel free to share some of your thoughts, and what not.