Frank Miller the author of 300 wants to write a sequel to the 300 movie. Alan Moore the author of Watchmen doesn't want to have anything to do with any film, period. It's possible for them to write a sequel without the author, but that would be complete ^^^^, I doubt they'd try it.
Anyone who knows anything about history knows there is no 301. They might as well make a sequel to Watchmen with that attitude.