As related by Miller to the LA Times, "the story starts with the Battle of Marathon, which was killer to draw, by the way, even if it was a lot of work," Miller said. "The lead character is Themistocles, who became warlord of Greece and built their navy." The potential for a good story is certainly present, provided that Miller can avoid some of the excesses that marred 300--which he calls "a deliberate propaganda piece:"
When I work on a story I choose a point of view. For this story, the approach was to tell this story the way the Spartans told it around the campfire. That's the reason they were fighting against 80-foot elephants and that's why Xerxes was portrayed as much larger-than-life figure and given these traits that the Spartans would [project on to] their enemies.
Miller isn't designing Xerxes explicitly for the big screen, claiming that "I don't do a comic book thinking there is a movie. I just want it to be as good a comic book as it can be. It's up to Zack and company to make it work as a film." His former partners are intrigued:
[300 director Zack] Snyder and "300" producer Thomas Tull have seen some of Miller's completed pages and plan to pursue it as a feature film if the finished tale lives up to their hopes. "If the book is awesome and compelling," Snyder said Monday, "then, yes, we're interested."
Given the fact that 300 "set box-office records for a March release and became the highest-grossing R-rated film of 2007," their enthusiasm is not surprising. MovieLine said "I think we all know a movie adaptation is inevitable," which will be fine by me as long as Snyder refrains from adding any deformed executioners or corpse-festooned trees.
Wikipedia's page on Xerxes is a good place to start