Dark ending?

If the writers were nasty they'd end it this way.

They'd cut back and forth between the weirdness spreading out from Gravity Falls and people fleeing from it and calling for help and our heroes fighting Bill in Gravity Falls.

They they would cut back and forth between our heroes fighting bill and the government in Washington DC trying to figure out what is going on and what to do.

Then they'd cut back and forth between our heroes fighting Bill in gravity Falls, the army fighting Bill's minions and the weirdness at the edge of the weirdness, and the president deciding whether or not to launch thousands of nuclear bombs at the the area of weirdness and Gravity Falls in particular.

Then they'd show our heroes finally defeating Bill and saving the world and restoring the old gravity Falls. And all the survivors would start celebrating wildly. And then thousands of atomic bombs would explode overhead, vaporizing every body and everything.