We reported back in October that Zack Snyder is directing Superman for Warner Bros. There is speculation that the Superman script needs more work, especially in the third act. It is also rumored that Zack Snyder's latest movie Sucker Punch is testing poorly, which may be why the studio is offering Xerxes to another director.
The last we heard on Xerxes was back in September, when Zack Snyder revealed he was halfway done with the script. We also reported in July that Zack Snyder is writing that script along with Kurt Johnstad. Here is what Zack Snyder said about the plot to Xerxes, which he called neither a prequel or a sequel to 300.
"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]. 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."