As an original Gen1 Transformers fan, I thought I'd put my penny's worth idea in for the fourth film incarnation.
How about this... Reactivated, 1939 World War 2 Combaticons (evil Nazi-bots from Cybertron) disgruntled after there 1942 annihilation start an onslaught on the Earth (present day) and every Transformer there.
The Autobots and Decepticons temporarily join forces to retake Earth with help from an old WW2 robot and Cybertronian friend of both-sides. The question is can the alliance last long enough to save the planet?
Yes, there's a geek in us all.