This is just a story idea I've had in my head. Not sure where the story would go from here, but I always figured it would be the easiest way to start a crossover between Star vs the forces of evil, Gravity Falls and Steven Universe. Set after the final battle for Mewni episode, between Lion 4 and Doug out, and after Weirdmaggedon.
Star and Marco have just been on one of their inter-dimensional adventures. Things are slightly awkward between them, but they are trying to ignore it. However, when they open the portal back to Earth, they end up in the forest next to a weird statue that Star names "The one-eyed top hat." They try to portal back to Echo Creek, but the portal doesn't work. Star tells Marco how magic is on the fritz, and so they were sent to the weakest point in the Earth dimension's barrier. This means that scientific laws from earth are being mixed with scientific laws from other dimensions, further proved when they see gnomes similar to ones they have seen in other dimensions, yet these gnomes have no knowledge of these other places. Figuring they are stuck there for now, they decide to explore.
Meanwhile at Beach City, the gems, Steven and Connie are preparing for a mission (Lion's coming too, but he doesn't exactly need to prepare himself). They recently noticed a pattern among the corrupted gems, with it being that those who are in a certain radius of a town called Gravity Falls (they only knew a town was there when Steven and Connie got back from talking to Ronaldo and asked if they could go to the place, showing where it was on the map) are, at different paces, moving towards the area. Deciding to investigate, and deeming it safe through the use of future vision, they warp as close as they can only to be attacked by a passing Gem monster. They defeat it, but Garnet points out something is wrong with her future vision. Although they don't want to admit it, with the exception of Steven and Connie, all of the CG's feel different in a bad way.
Basically, the gems, Dipper (who has decided to remake the journals after they were destroyed, with help from Grunkle Ford on video chat), Mabel, Star and Marco have some sort of adventure. Maybe they fight Eclipse? I don't know. The problems they face are the fact that the Gems are being affected by magic being on the fritz (this could have been the reason why Homeworld is running out of resources and therefore why Peridot didn't have powers) and therefore some of their powers are unreliable, namely Garnet's future vision and their shape shifting, which annoys Amethyst. Star's wand is as strong and safe as it used to be, but constructs she makes don't last long. The Pine twins notice that there is less weirdness than before and Lion's portals, when opened, keep growing out of control. He is also becoming less pink...
How much is caused by magic, albeit of a different kind to the gems, being on the fritz, and how much is caused by the different forms of science overlapping in one place? Who are they trying to defeat? And can they solve it before Marco's parents get there? Fun little bonus, the alien that either caused the weirdness or was attracted to it was unknown to the gems as well.
Some facts that help clear things up, Connie's parents are deciding to allow her to do her thing as they continue to try and be more supportive of what she wants to do, while Doug feels like he isn't cool enough. Marco's parents found out he was there and are coming in an attempt to be more involved in their life, given they know almost nothing of what he does during his time. Greg is fine as long as Steven promises to stay safe. Stanley and Stanford aren't there; they're still travelling the world. They have a website they post their findings on which Ronaldo is a member of. It's thanks to their findings that he knows so much about the gems.
For another idea, it could be that the Stan o' war docks in Beach city to investigate what Ronaldo put's on his blog and the two ends up meeting the gems after engaging in a fight with a monster. That one seems to hold less potential because there is no good reason for Star and Marco to be there.
Anyway, if anyone takes this up, please PM me so I can read it. Personally I don't know what to do with this; it just seemed interesting how easily they could all line up. Weakest point in the dimension, added to the recently closed tear in space-time courtesy of Bill Cipher, plus Gem monsters go to magical places? Heck, the torch that makes things bigger and smaller could have been a piece of a long dead gem, whose powers were deemed useless by the Diamonds in the war due to the ability to shape shifting powers, or the need for sufficient light and concentration or something (given that when they use the torch the beam gets wider, which makes it more of a close up thing.
Anyway, I hope someone used this, so see you later!