Well, someone finally asked a 300 alumni other than poor Gerard Butler. IESB.net caught up with director Zack Snyder, who revealed that 300 would receive the sequel treatment. Snyder did the impossible, and actually spoke to Frank Miller about it, and learned he's writing a graphic novel that takes place between Thermopylae and the Battle of Plataea which is seen at the end of the film. There's a mere year in between waiting to be populated with new Spartan heroes (though David Wenham's lone survivor, Dilios, could return) by way of Miller's pen. Snyder promises to direct the adaptation when Miller finishes the book.
There's certainly historical material to draw from -- the time between Thermopylae and Plataea was marked by several battles -- the naval Battle of Artemisium, which occurred alongside Thermopylae, and the Battle of Salamis. Both were victories for the Greek states (although Artemisium could be argued to be a draw), but not without cost. Several Greek cities, including Athens, suffered severe Persian attacks.
But while the Greco-Persian Wars are ripe for many stories and movies, I don't see any of them fitting the hyper-stylized mold of the original graphic novel or the film. The insanity of it all worked because it had a legendary story and king to anchor it down, and shine through the blood-splatters and giant rhinos. Why water that down? As fond as I am of Snyder, Miller and violent boys in leather pants, I wish they would just let the Spartans lie.