Season finale will be Weirdmageddon Part III, which will air who knows when? A week? A month? A year? Ten years? Hiatus between episodes is even worse than My Little Pony, which had a several-month hiatus in the middle of a season once!
What My Little Pony has going for it is, as a kid-friendly show the heroes are always guaranteed to win and only the most evil villains can die. So the Season 5 finale will turn out all right, but I honestly don't think the Gravity Falls series finale is going to.
Series finale will have Dipper and Mabel go up against Bill Cipher, who has the powers of a god. He is even more powerful than Discord from My Little Pony, who was turned to stone by the Elements of Harmony and was later reformed. But there are no Elements of Harmony in Gravity Falls, are there?
I thought Bill died when he possessed the puppet and Mabel blew him up, and everyone who said otherwise was another instance of "I think this villain survived" (like we got with the Deadly Six in Sonic, Queen Chrysalis in MLP, even King Sombra in MLP who was pronounced dead by the episode's writer). I couldn't have been more wrong. He returns to finish the series with a bang! He's invincible!
So as far as I can tell, the plan is to send Bill back into the dimensional rift from which he came. But first, can they? Is it even possible? Second, even if they do send him back, he got out once, who's to say he can't do it again? Like, back to MLP, King Sombra was banished into a frozen prison, but he returned on his own, which is why Princess Cadence and Spike had to kill him. Even though Tirek was imprisoned back in hell, he has a cage around him to prevent his re-escape, and that's assuming his second imprisonment wasn't damnation (he gets blasted by a rainbow, and suddenly appears in hell?).
So, since we know this is series finale, who's to say Dipper and Mabel make it out of the series alive? I think Bill is going to win and destroy the earth, turn it into a chaos capital, whatever his grand scheme is. He simply cannot be defeated.
Besides, fans today want villain victory (though I'm not one of them), and the only villain victory I am more certain of than Bill is Thanos in Avengers: Infinity War