Okay, this is awesome. Given that I am also working on a giant multi-fandom crossover of my own, I have so many questions to ask! Or just chat, it depends on your free time. I don't want to bother :c
First off, I'm curious about your plot. I tried to make some research, but aside from explosive action scenes and trailers, I didn't see any actual explanation as to what exactly is going on; and though some details in the trailers led me to think that there definitely is more to this than just "putting cartoon characters together for the fun of just seeing them together", well, I couldn't find the episodes really explaining how the characters meet and how they unite or whatever.
(I'm saying this because, well, this is basically among the most important plot points in my own crossover, to me the relationships between them and how they investigate together (or against each other...) is much more important than action in itself.)
Second, this is probably a silly question, but are you working on all this alone? Because if so, congrats, I don't think I could draw all the characters with so many different drawing styles, let alone animate them, work on the plot, maybe remix some musics and all that.
(My crossover is a 80% fanfic 20% comic story, and I spent over five years working on the background plot.)
Anyway, given that you look so experienced and everything I couldn't help ask for little tips... I hope I didn't bother ^^'
[EDIT] Okay, so after watching practically all of your available videos, I did find one where you supposedly explained everything orally x) Unfortunately I'm just a 19-year-old French girl so even if I watch all my cartoons in VO, my oral comprehension isn't perfect and I had struggles understanding everything-- from what I understood though, it really sounds like the characters just sort of meet "for no reason" and fight constantly against villains who attack the multiverse because they're villains. So yeah, classy and with lots of incredible action, but not as much plot as I hoped ._. I may have misunderstood though, and in any case it doesn't make your work any less impressive! Just that, well, I personally prefer when action serves the plot, not the opposite. At least some explanations as to why Star, Steven and everyone else just so happened to be in Gravity Falls "coincidentally" at the same time as Dipper and Mabel, for example :')