Ummm, there was only one 300. Only one. It's not fiction, it's historical fiction, and you A) Can't change history and B) It's not the year, it's the number of spartans at the battle.
Hmm about about 0? Since that's how many Spartans were left
Incorrect. One was left. The one who lost his eye and returned home to tell the tale. They can't make a sequel. Unless as stated before by others. The other armies that had to do with the battle.
It would probably be another parody of 300. Maybe another kind of Meet The Spartans with a different name because as many people have said, you can't remake a sequel of a movie based on a historic event.