The first two transformer movies have been set on Earth where the good Autobots basically try and defend Sam and the humans from the evil Decepticons so to allow humanity their rights of freedom as being sentient beings. That's what makes humans being, you know.
Fights in forrests with Optimus taking on Megatron, Starscream and that other dude are awesome enough but what we need now is interstellar space battles. Transformers 3 should take place in space as much as possible.
So what does one do in space? Visit planets. Visit home worlds like Cybertron. Learn about the origins a little more.
The third movie needs to have the Autobots searching for something they need to SOLVE THE PROBLEM.
In movie parlance, this is the McGuffin, a the plot device to make things tick over to help solve the PROBLEM. Every film needs a McGuffin. For the first animated movie it was the Matrix of Autobot Leadership. The second had the All Spark, the third some kind of variant on the Leadership Matrix.
The Autobots should be looking for the The Key of Light in space, on a new planet or on Cybertron itself. Lets call it The Key of Light.
But what is the Key of Light for? It solves the problem. And would the problem be? In the end there can only be one problem, and his name is UNICRON. The Shiva of the Universe. Destroyer of Worlds. The biggest bad ass robot there is.
So the threat would be that Unicron is going to destroy Earth / Cybertron / Solar System whatever.