Thought id finally jump in here... The Rumour mill is hard at work, but a few solid facts have come through.
Yes there is going to be sequels... Plural.. A trilogy at the least. Megatron will come back for one more movie. Now this is where you need to go back and watch the movie very closely again, as these next facts were hinted and finally revealed by the producers and writers of the movie...
The Original Series Transformers could only take shape once, however the writers became creative and decided that in all fairness, the Transformers should be able to change form to suit them in what they need, by scanning an object and duplicating it, shown by Bumblebee when he 'Upgraded his Camaro image". It was announced in a conference that this decision, which would seem practical in todays society, was also in case of possible sequels, to aid in the continuity of the series.
Secondly, in the middle of the final battle, 2 Decepticons went AWOL... Starscream and Barricade.. Everyone who has watched through the credits knows about Starscream doing a runner, but nothing was said/shown about Barricade... This is where the announcements stop however and the 'Mill' come into play...
Its mostly believed that Starscream goes for re-inforcements.. (its a given) but when they show megatron and the Decepticons being dumped into the Laurentian Abyssmal, they never show a underwater sequence showing his disappearing into oblivion.. Given the fact that the Transformers can change into what suits their needs, this would be where Barricade comes into play... The thoughts are that the next movie will start with Barricade scanning a submersible rescue vehicle and secretly following the US Navy to the dropoff point, and rescuing the remnants of the fallen Decepticons...
Just to finish off this post, unconfirmed but appealing rumours of the appearance of the Insecticons in the next movie have surfaced, with a possibility of a Dinobots/Beast Wars hybrid for the last one...