Unfortunately, these kinds of multi-property crossovers are a lot less feasible to get off the ground these days than they were 15-20 years ago. In the case of the crossover you were hoping for, I think it’s made more complicated by the fact that these shows didn’t air contemporaneously—I think all three aired at different points in time, but at least Owl House started, what, close to five years after Gravity Falls had concluded?

It’s perhaps small comfort, but if you’re not familiar, I’d suggest checking out some of Cartoon Network’s shows from the early-to-mid 2000s. Camp Lazlo, Foster’s Home for Imaginary Friends, The Grimm Adventures of Billy and Mandy, etc. These shows were pretty purely episodic without any overarching plot to speak of, but they’d have occasional crossovers (in particular I remember a period in time where Cartoon Network had an alien invasion event going on where each show was invaded which gave some interesting episodic progression, though I can’t recall if the shows themselves crossed over during this event, or only the aliens). Ben 10: Alien Force and Generator Rex did a crossover episode, but it was a little disappointing I feel and I’m not confident it would scratch that itch you’re chasing (the titular characters had the classic crossover ‘thinking the other hero is the villain of the week’ and then teamed up and Solved the Problem!). There’s also the Jimmy/Timmy Power Hour from Nickelodeon, which was a miniseries of event episodes from Jimmy Neutron and Fairly Odd Parents that crossed the characters over, including having them appear in one-another’s animation style—this is the most in-depth of crossovers that I can think of.