What are some interesting plot ideas for a TMNT Undertale crossover fanfiction?

One idea could be that the Turtles from 1987 end up in the 2003 universe and have to adapt to the darker and more serious tone. They could team up with the 2003 Turtles to fight a common enemy like the Shredder. The 1987 Turtles' more light - hearted and comical ways might initially clash with the 2003 Turtles' brooding nature, but they eventually learn from each other.

A possible plot is that an ancient evil that was sealed away in the 2003 universe is accidentally awakened by the arrival of the 2012 turtles. The 2003 turtles initially blame the 2012 ones but then realize they need to work together. They combine their strengths, with the 2012 turtles' modern thinking and the 2003 turtles' deep knowledge of ancient ninja lore, to defeat the evil.

One idea could be that the Turtles from 2003 get transported to the 2012 universe and they have to deal with the different versions of their enemies. For example, the Shredder in the 2012 version might have some new powers or tactics that the 2003 Turtles aren't used to.

Well, another plot could involve Sans and Papyrus coming to the RWBY world. Papyrus might be fascinated by the Grimm and try to befriend them, while Sans could use his powers to help the RWBY team in their battles against Salem. He could also share his knowledge of timelines with Ozpin.

Well, it could be that there is a glitch in the multiverse that causes a fusion of the two worlds. So we have characters like Frisk interacting with Freddy Fazbear. Frisk might try to show the animatronics a different way of living, more like the pacifist way in Undertale. And in the process, they all have to deal with the threats that exist in both worlds, like the purple guy from FNAF and the more malevolent monsters from Undertale.

A possible plot could involve the souls in Undertale having some connection to the spirits that are rumored to be in the FNAF animatronics. The main character from Undertale, perhaps Frisk, could enter the FNAF world and try to free the trapped souls. In return, the animatronics could help Frisk in the Undertale world against some powerful foes.

One idea could be Sans from Undertale ending up in the Marvel universe. He could use his shortcuts and bone attacks to fight alongside the Avengers against a common enemy like Thanos. His laid - back attitude would contrast with the more serious heroes in Marvel.

One idea could be that the turtles from 2012 somehow get transported to the 2018 universe. They could be really confused by the different art styles and the new gadgets the 2018 turtles have. The 2018 turtles, on the other hand, would be intrigued by the more traditional look of the 2012 turtles. This could lead to some funny moments as they try to communicate and figure out how to get the 2012 turtles back to their own universe.

One idea could be that Danny's ghost powers start to malfunction in the TMNT's dimension, causing strange ghostly events that the turtles have to help him fix. Another could be a villain from Danny's world teams up with one of the TMNT's foes, forcing the two groups to work together. Maybe Danny accidentally gets sent to the sewers where the turtles live and they initially mistake him for a new type of mutant.

An Undertale and My Hero Academia crossover is also a great concept. The heroes and villains in MHA could find themselves in the Underground. Characters like All Might could have interesting interactions with Asgore. And the idea of quirks in MHA and the unique abilities of the Undertale characters could be a really cool aspect to explore, like how Bakugo's explosive quirk might compare to Sans' bone attacks.