Zack Snyder had this to say about the sequel's possible plot.
"There's something that happens in history between Leonidas dying at the Hot Gates and Platea," Snyder explained of the possible plot, mentioning the events between the Battle of Thermopylae and the Battle of Plataea. "That's a year that's left out of the [original] movie. A lot happened."
On whether or not a follow-up movie will be developed, Snyder claimed, "Frank is definitely working on an idea. If Frank comes up with a great idea and draws something cool, there's no reason why we wouldn't make another movie." Asked further whether he would return to do another "300" film, he said, "If Frank does something great then yes. It's on him now."
"300" is a 2007 epic battle film that chronicles the story of King Leonidas and his 300 Spartans who fight to their death against Xerxes and his army
I don't see the need for a sequel. I suppose if Millar is writing, it will be good. I hope that Zack doesn't try anything like this with the Watchmen.