It might be just me, but I think 300 is a fairly self-contained film. The adaptation of the Frank Miller graphic novel retelling the Battle of Thermopylae, where 300 Spartan soldiers faced off against Persian king Xerxes million man army, pretty much cover all there was to the event. But according to Zach Snyder, a sequel is well underway.
Miller, never one to pass up an opportunity to despoil his own legacy (Too harsh? He did give us The Dark Knight Strikes Back, you realize), is writing a comic sequel to his original graphic novel. So says Snyder, making the rounds to promote the Watchmen DVD and the 300 Blu-Ray releases.
It looks like the sequel will be broader in scope, showing what was going on in Athens and on the Aegean while the events of the first movie took place.
Once the graphic novel is completed, Snyder presumably will helm the film adaptation, using the same green screen process he used on the first film.
