It ended too soon
Gravity Falls was such a great show, but it really ended far sooner than it ought to have. I think that was mostly because of the creator losing interest and wanting to go do something else. Still however, it really needed at least one or more season.
So many untapped ideas for future exploring of the town, meeting new characters, and having new enemies besides Bill and Gideon could have been implemented. Wendy could have had more episodes of going on adventures with Dipper and Mabel. Robbie could have been given some moments to shine and be a hero. Hell, Pacifica was on her way to becoming a fully fleshed out character before the show ended. We could have seen her bond with Mabel and rebel against her controlling parents, while still being the bossy rich girl.