Guild Wars follow-up, it seems. Superannuation recently dug up a pair of trademarks for "City of Heroes 2" filed by the studio on April 1. As with all video game trademarks, it's possible NCSoft is just reserving the name and may not have plans for a sequel at this time.
Alternatively, the company might just be pulling an extremely discrete April Fools' Day joke, though it already pulled that off with the recent announcement of City of Sidekicks. We've contacted NCSoft to see what we can find out.