Well, if a girl falls into DC Universe fanfiction, she'd likely be plunged into a world full of chaos and heroism. There are so many different scenarios. Maybe she'd end up in Metropolis and become a sidekick to Superman, helping him save the city from Lex Luthor's evil plans. Or perhaps she'd find herself in the middle of a fight between the Justice League and Darkseid, having to figure out how to survive in this dangerous and exciting world.
She might meet Batman himself and become his sidekick. Maybe she has some special skills from her own world that can help Batman in his fight against crime in Gotham.
No, it's not ethical. Naruto is a story that is aimed at a wide audience, including young viewers. Incestuous themes are not only against the values of the Naruto world but also inappropriate for the target audience. The ninja clans in Naruto have their own traditions and rules, and this kind of relationship would be a violation of all of them.
One possible plot could be that the Gems come to Gravity Falls to investigate some strange energy readings that are similar to their own gem powers. They meet Dipper and Mabel, and together they figure out that there is a connection between the two worlds' magic.
The unique worlds of both shows. Gravity Falls has its mysterious and spooky elements, while Steven Universe is all about love, acceptance, and gem magic. Fanfiction allows fans to explore these worlds further. For example, in Gravity Falls fanfic, people can create more adventures for Dipper and Mabel. In Steven Universe fanfic, more relationships between the gems can be explored.
You can start by looking on popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of Gravity Falls and Steven Universe fanfics. Just use the search function and type in the relevant keywords. Another option is FanFiction.net, which also has a decent number of such fanfictions.
One possible plot could be that Steven and the Crystal Gems end up in Gravity Falls. They could team up with Dipper and Mabel to solve a mystery that involves both the magic of Gravity Falls and the Gem technology. Maybe there's a Gem hidden in Gravity Falls that has been causing strange phenomena, and together they have to find and deal with it.
A popular pairing could be Dipper and Steven. They are both curious and adventurous, so fans might like to imagine them exploring new worlds together. Their different backgrounds would make for an interesting dynamic.
One interesting crossover could be the characters from Gravity Falls and Steven Universe teaming up to solve a mystery that involves both the Gem world and the strange happenings in Gravity Falls. For example, Dipper and Steven might combine their knowledge of the supernatural to figure out a new threat.
It could change the dynamics between the characters. For example, it might make Asuma a bit jealous if he were still alive, creating some tension. Also, Naruto's friends might be a bit surprised at first but then support him. It could also lead to new alliances as Kurenai's friends and family might be more inclined to support Naruto in his future endeavors.