Puck, they could easily make a sequel.
Megatron might be dead... but there were other villains.
Starscream took off at the end so he can go find more decepticons and bring them to earth.
He could find/create Galvatron (in the orig. comics galvatron was created from a destroyed megatron... we have a destroyed megatron now).
There's always Unicron, Shockwave, Soundwave, Scourge, the contructacons forming Devastator, insecticons...
Either way the film is a box office hit beating records set by spiderman 2. I'm sure they'll figure out a way.
And if you think way back to the original... the transformers crashed on earth looking for energy to revitalized there planet. Allspark is gone now so they'll have to find alternate sources... plundering earth and getting revenge on the autobots/humans seems like a good motive to bring them back to earth.