I know what you're thinking: what can you really do to make a 300 sequel.  The answer is easy...don't.  The new story will feature characters from the film but will remain it's own self-contained story.  They just both happen to center around the same war (it's a big war so there's plenty of wiggle room).  With Frank Miller still in the process of writing the new story, Xerxes, it seems odd that work on the film adaptation has already begun.

Snyder has confirmed in an interview with LA Times, that they've already worked out a writing deal for the film and has been working on the script for the past week.  Like on 300 Snyder is again working with Kurt Johnstad to adapt the story for the big screen. Even more amazing that he's starting writiing a script for a comic book that won't come out for at least another year is the fact he's not sure what role he'll play in the filmmaking process.

While he has a first-look deal in place with Warner Bros. no deal is in place for him to actually direct the new movie.  If I were a betting man, I'd go ahead and assume that he will be sitting in the director's chair, since he's already so intensely involved in the project.  Time will tell, but until then here's what he teased fans with for the story of the new film:

"This movie follows Themistocles and the Battle of Artemisium, which coincidentally happens on the exact same three days as the Battle of Thermopylae [which was the basis of '300']," Snyder said. "This one starts off with a quick retelling of the why of the Persian wars. It starts off at the Battle of Marathon and then it goes back to Themistocles finding out that Persians are invading again. and off we go over to learn a little bit about why Xerxes is the way he is. Darius [the Persian king and father of Xerxes] gets wounded at Marathon and he's super cool and like a great guy. Even the Greeks are like, 'Darius is awesome.' After Darius dies, Xerxes is so distraught, but Darius had told him, 'Don't attack the Greeks, only a god can punish the Greeks.' So that's when he calls his mystics and wizards and says, 'Make me a god so I can avenge my father.' "

-Jordan