1. While the finale ended the main Bill story arc, it also left the door open for Mabel and Dipper returning to GF next year or many years later.
2. There are big hints that Bill is still alive and wants revenge. Why end the series with that? That's a bad idea. Maybe there's a plan to bring him back.
3.Disney would probably want to pay Alex a lot of cash to to continue GF somehow as it was a big critical and financial success nd studios want as much of those as they can get.
4. Alex said in an Entertainment Weekly article that it's possible that the show and characters may return somehow.
5. Considering Alex still tweets a lot about Gravity Falls and the characters after the show ended and trying to work on other film and televison projects, I gotta feeling that after a few years, he'll miss the Pines Family a lot and miss making new GF episodes, and he'll return.
6. Jason Ritter and Kristen Schaal have said that they would return as Dipper and Mabel in a sequel series.
7. With the huge success of Journal 3 and it becoming a NYT bestseller, I'm positive that has had Disney and Alex talk about the future of the show. Studios love money. And big successes like the Journal can change the original plan. We'll see how the special edition sells,and if it's a hit, that would increase the possibility.
8. With Disney buying Netflix and probably developing new shows for it as well as reviving old ones, they could put a new Gravity Falls series on Netflix. That could also work in Alex's favor as Netflix gives you more creative freedom.
9. Gravity Falls has become more and more famous and beloved since it's premiere and end. More and more people are now watching the show and praising it and more of the Pines Twins. Disney has to think about that.
10. A lot of people thought season 2 part 2 felt really rushed and were disappointed that some character and relationship arcs weren't finished and some stuff wasn't developed well. Another GF show could make up for that.
11. I don't always believe creators when they say the world they created is done. Lucas said that about SW, it came back. Stallone said he was done with Rocky, it came back. Ducktales was done, it came back. Gilmore Girls just came back, Will And Grace may come back, Jimmy Neutron may com back on Netflix, and there are many other examples of movie franchises and TV shows that the creator said would never come back and they did.
12. For people who don't want more GF because it ended and we don't need more, think about this. Do you miss the characters? I know most fans do. It doesn't matter what the plot is. We just love Dipper and Mabel. And no movie or show needs a sequel. Star Wars didn't need a sequel, but we're all glad Empire Strikes Back happened. Godfater didn't need a sequel, but we all love Godfather 2.