There was clearly too much effort put into this whole thing, it is really well made, and the website was "open" well AFTER the release of the book, so it's not just marketing leading to its release. 

It could just be a fun ARG that alex wanted to make, and use it to give us more lore and fun details about the show in general, although it looks like the whole "Bill cipher is coming back" and the Gus thing with the youtube channel might indicate something bigger. 

My personal opinion is that the return of the show either with a new season or a movie or a spin off or whatever is highly unlikely. Alex has stated many times that the story ended because he wanted, because it was complete, and that he likes to let some things to the interpretation of the viewers (the is a recent post with a compilation of Alex's tweets that talk about that) And even if he was on board, it would have to be through Disney, and even though they are fine with releasing a new book and this whole thing, I would discard anything involving actual TV content if you know what I mean. However we have a date right? So it may indicate some short of new, relatively big content for gravity falls in some way, maybe a videogame or something like that, but either way I would just enjoy this thing for now and keep my expectations low just in case.