My disclaimer is here. I don't own anything.
Plot ideas for whoever wants them! Some spoilers within, but you're on a fanfiction website. Did you not expect spoilers when you got here? Idk what to do with these, use these however you want to. No strict guidelines for any of these, just suggestions. Suggest you post on archiveofourown for the ideas with more than two fandoms involved. Some of these are more suited as one-shots and some of these can apply to different fandoms, these ones just lined up with my ideas for them. Some of these are just crack.
My Hero Academia: Quirk: Scribblenaut, any object/noun you write in any notebook comes to life. The notebook can not read intent, so you have to be careful for double meanings. Any notes for class have to be taken on loose leaf paper to avoid accidental creation.
Rise of the Guardians: Jack and Sandy are friends before the events of the movie. Sandy also has not told the rest of the guardians about their friendship and has something to say about the blizzard of '68.
Pokemon: narcoleptic pokemon trainer's journey through whatever region/regions.
Pokemon: play a game of Gym Roulette, switch around all the gyms or gym types. Examples: give Brock the fire type or put Lt. Surge's gym in Cinnabar.
Fairy Tail: Gray being an ice dragon slayer instead of trained by a human.
Undertale: handplates AU combined with PTA Sans. Basically, they're on the surface, but Sans and Papyrus have/had handplates.
Undertale: hypersomnia and/or insomnia PTA Sans.
Undertale: Frisk traveling through the Underground backwards, starting at Asgore's castle and ending at the beginning of the ruins. Characters all live in the same places and Sans is Sans so you can meet him whenever. Try to keep the characters' personalities.
Undertale: schizophrenic Sans going to therapy on the surface. Genocide run happened right before this one. Maybe you can put him on the pta, your choice. I just really like those AUs.
Undertale: Sans and Papyrus being experiments (maybe a handplates or baby blaster type of thing) and going to live with Toriel, Nice Cream Guy, or Muffet. Grillby's starting to get overused.
Undertale: Sans has a nightmare and starts sleep fighting. Can happen in any combination of Undertale AUs. Put Dusttale Sans in Swapfell or something if you want.
Undertale: Papyrus or Monster Kid accidentally trips on Napstablook.
Minecraft Diaries: Daniel and/or Travis living in Phoenix Drop. Can happen before or after the 15 year timeskip in the case of Daniel, for Travis it must be before. Must have adventures with Aphmau, but if they stay in the village for an adventure or a few it can either follow Aphmau, Travis/Daniel, or you can switch povs.
Minecraft Diaries: Zane knowing Travis before he meets him in the series.
Young Justice: Wally doesn't know Robin before they form the team.
Naruto: Kakashi raises Gaara. Not biologically related, can take place in any village.
Naruto: transgender Shino or Shikamaru. Can be a thing where an oc gets put into him, like how people do for Sasuke a lot. Like, they die and then get reincarnated into the Narutoverse as Shino, but they were a girl in their past life and still identify as such. I don't want a sibling for them, I want them to be the character.
Monster Prom/Undertale Crossover: turns out that not all monsters were forced into the Underground.
Steven Universe/Detroit Become Human Crossover: Steven is a cyborg made from Rose's parts after she got outed as a deviant and she was destroyed. Steven also has at least part of her memory chip. Greg is the one who went and got Rose's parts from the junkyard. Pearl is a deviant from a housekeeping android, Amethyst could be one that was made for fighting or something like that, and Garnet is an android with two memory chips. They also either start out or end up living somewhere without android laws. If they are living somewhere like that at the beginning of the story, tell how they got there through flashbacks, but it doesn't have to be all at once or right away.
Young Justice/Undertale Crossover: monsters come to the surface, Batman is incredibly suspicious of them, and Frisk, Chara, Monster Kid, and Asriel go to Keystone High School (Wally's school) or Gotham Academy, where Toriel becomes a teacher. You can put other monster children in there if you want.
Naruto/Undertale Crossover: monsters surface, create the Village Hidden in the Ground (they still use the Underground), Papyrus becomes a diplomat, Frisk becomes a diplomatic ninja on a team with Monster Kid and anyone else you want, Sans is basically their version of a Nara, Undyne is the team leader, Alphys is part of the council, Mettaton and Napstablook live together maybe, whoever you want to lead the village leads it, and they hold/join their first chunin exams.
Undertale/Camp Camp Crossover: I'd like to see Sans as a councilor at Camp Campbell. And Frisk and maybe others can also go to their own personal camps. Maybe a few can join the other camps.
Undertale/GTA 5/Fake AH Crew Crossover: the monsters living in Los Santos and one of them meets a member of the crew. Basically every important monster except Toriel, Asriel, Asgore, Frisk, Monster Kid, and Chara join the Fake AH Crew and works with the main 6/7 (depending on if you have Ray, Jeremy, or both of them). Could also work with the Underfell AU. I suggest posting this on archiveofourown so that you can have all three fandoms listed.
Undertale/Fran Bow Crossover: Fran falling into the Underground instead of Frisk (at the part of the game when she falls into Ithersta would work) or both of them falling into the Underground. With Mr. Midnight.
Undertale/RWBY Crossover: any combination of Sans, Papyrus, Frisk, Chara, Asriel, Alphys, Undyne, and Monster Kid being on two separate teams. Maybe you can have them break the barrier and have them be a trial run for future generations of monsters becoming hunters and huntresses. Frisk can still have ambassador duties and this starts at the beginning of RWBY.
Undertale/Pokemon Crossover: Papyrus and Sans are brother Cubones/Marowaks (Sans as Alolan Marowak and Papyrus as regular Marowak) and they end up getting the same trainer. Sans can still teleport and I guess you can have use aura or something for their blue magic.
Undertale/Pokemon Crossover: Sans as Ash's Pikachu. A lot of people make Pikachu drink ketchup, so why not? Maybe make his electricity blue.
Undertale/Fullmetal Alchemist Crossover: Sans is a state alchemist (could be skeleton or human) who lives with Papyrus, who is either a state alchemist or something else. If you keep them skeletons, you could attribute it to human transmutation or something else.
Undertale/Detroit Become Human Crossover: they reach the surface and eventually the monsters find out about the androids. You can decide whether or not the androids have souls, but deviants should still have feelings.
Fairy Tail/Pokemon Crossover: every pokemon has it's own celestial key. You can either make it so that people in the pokemon world have to use keys instead of pokeballs or you can just give a celestial mage a bunch of pokemon. Maybe make Happy a shiny mew if you want to, or some other blue cat pokemon.
Fairy Tail/Undertale Crossover: the monsters being celestial spirits, Frisk and Chara share a key. Give them summoning conditions that suit them.
The villains are really protective over one very specific person, could be anyone, and the heroes notice.