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 the Ninja from Ninjago are transported to the TMNT's world. They team up to fight a common enemy like the Shredder who has somehow got hold of some dark magic from Ninjago. The Ninja can show the Turtles their unique fighting styles and the Turtles can introduce the Ninja to the sewers of New York as a base.
One challenge is staying true to the characters from both series. The 2012 turtles have different personalities compared to the 2018 ones. For example, the 2012 Leonardo is more strict about following rules, while the 2018 Leo is more laid - back. You have to make sure that when they interact, it makes sense based on their original characterizations.
The plot could involve a new threat emerging that affects both worlds. This threat could be a powerful sorcerer who wants to combine the technology of the TMNT world with the magic of Ninjago. The Ninja and the Turtles would have to put aside their differences and work together. They would travel between the two worlds, exploring each other's unique environments like the Ninjago Monastery and the TMNT Lair.
One major theme is the idea of family. The TMNT are a tight - knit family of mutants, and Batman has his own sort of family with Robin and Alfred. Fanfictions often explore how these two different family units interact. There's also the theme of justice. Batman has his own code of justice in Gotham, and the TMNT have their own ideas about right and wrong in New York. When they cross over, they have to reconcile these different views. Additionally, the theme of secrecy and identity is present, as both Batman and the TMNT have to hide their true identities in their respective worlds.
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.
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 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.
The 1987 Turtles would probably be very curious about the more serious and intense 2003 Turtles. They might crack jokes at first to ease the tension, while the 2003 Turtles could be a bit annoyed by their silliness. But as they face challenges together, they'd start to respect each other. For example, when fighting Foot Soldiers, the 1987 Turtles' acrobatic and fun - loving fighting style could complement the 2003 Turtles' more strategic and brutal combat.
One popular 'tmnt and batman crossover fanfiction' is 'Batman Meets the Teenage Mutant Ninja Turtles'. In this type of fanfiction, you often see really interesting team - ups. For example, the turtles' unique fighting styles combined with Batman's detective skills make for some exciting storylines. There are also some where they face common enemies together, like the Foot Clan and some of Batman's rogues gallery working in unison.