Remedy hasn't confirmed the trademark application, so there's a chance it's an elaborate fan hoax. The filing does appear to be authentic, however, given it links through to a Remedy account that was also used to trademark Max Payne, Alan Wake and Bright Falls, a six-part live-action mini series. Still, for now any possible sequel is still firmly in "rumor" territory.
Alan Wake was an unusual third-person action game. The titular character, a writer, quickly finds himself reliving a story based on a manuscript he doesn't remember writing. Pages are scattered throughout the world, teasing horrors and mind-bending dangers that the player will have to deal with. Enemies were townsfolk and objects possessed by a mysterious darkness that Wake could beat back with torches, flares and other sources of light. Managing ammunition and light was a novel experience, especially as light could be used for both offensive and defensive purposes.
It was the story, however, that earned the game a cult following and 4.5 million sales (as of March 2015), despite it going up against Red Dead Redemption at launch. Remedy was able to make two DLC chapters for the main game, the last of which ended with Wake writing a manuscript called "Return." His adventures continued in American Nightmare, a downloadable sort-of-sequel that shifted the backdrop to Arizona. There he battles through an episode of the fictional TV show Night Springs, which, in the credits, is revealed to be called Return too.
The final piece of evidence? Microsoft is offering Alan Wake for free through backwards compatibility with every copy of Quantum Break. Pre-order and you'll get American Nightmare too. A perfect opportunity, you might say, to remind people about a somewhat dormant franchise...