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.
Regardless of the speculation, it seems the studio wants to start both Superman and Xerxes around the same time, making it impossible for Zack Snyder to direct both projects.
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.
It isn't clear when production on Xerxes will start, but with this offer out to Guy Ritchie, it looks like the movie could start up sooner than expected.
300: Rise of an Empire was released March 7th, 2014 and stars Sullivan Stapleton, Eva Green, Lena Headey, Hans Matheson, Callan Mulvey, David Wenham, Rodrigo Santoro, Jack O'Connell. The film is directed by Noam Murro.