Between the ‘Greg Universe is Greg from Over the Garden Wall’ and the ‘Dipper and Mabel are the children of either Greg or Wirt’ theories, I’m surprised I haven’t seen any crossovers/AUs between all three with Dipper and Mabel being Steven’s cousins. The naming might be a bit tricky to figure out, perhaps Greg renamed himself, or maybe Wirt took his wife’s last name and Grunkle Stan is related on their mom’s side, but while the paranormal between the two doesn’t seem to intersect, it doesn’t necessarily contradict, either. (Though if Gravity Falls ever does have different aliens, it would probably have to become an AU)
Star VS the Forces of Evil might be able to fit in as well. I’m not very familiar with it, but maybe the dimensional travel she uses could work in a similar way to the portal. It doesn’t appear to have come into contact with Earth very often, and considering the mug through the portal that ended up in Rick and Morty, the Pines/Universe family’s dimension could also be a different version of Earth.
On that note, though it did finish a while ago, Scooby-Doo: Mystery Incorporated could also join them. The series finale involved them entering a new timeline and mentioned other dimensions, too, and we don’t see anything about what the new world is like beyond Crystal Cove and the fact that there are more mysteries to be solved.
tl;dr, It seems to me that a lot of paranormal/adventure cartoons could crossover together without disrupting much of canon, and I think it could be a pretty fun idea.