Before start writing this theory and first ever post on this site, I would like to say that this is just a theory, and there's no way of telling I'm right or not until Gravity Falls Episode 17 season 2 comes out.
Now that THAT'S out of the way, I'd like to also mention this a response to a way better and more thorough post by Eleanor Tremeer. That was mostly about Bill though. This a theory of what the episode might be about.
The episode is called, "Dipper&Mabel Vs The Future" there are many hints and reasons that Gravity Falls might be heading towards an apocalypse, or Bill is going to bring his nightmare dimension over the real one.
There has been many time traveling episodes, such as "Time Traveler's Pig" and "Blendin's Game", and that lets us know there IS way to travel through time.
I think that in the episode, Mabel and Dipper might get their hands on another time traveling device, go to the future, and see either
1. There's an apocalypse. ("But bear this in mind, a great darkness approachs, and one day, everything you care about will change."- Bill, Dreamscapers)
2. They end up like Ford and Stan. ("You don't think we'll end up Ford and Stan, do you?"-Mabel, Tale Of Two Stans)
Either way, I feel like they're going to find a way to prevent one of these events, hence the title, "Dipper And Mabel VS The Future".
That's all for now. Thanks for reading,