It could merge the main story arcs. For instance, if the 2012 turtles are facing a big threat like Kraang, the 2003 turtles could come in and offer their combat experience. Their different fighting styles would make for some exciting battle scenes.
It could involve the different art styles of the two series. For example, the 2003 version has a more angular and detailed look compared to the 2012's slightly more rounded and colorful style. Maybe in the fanfic, the turtles from each version could meet and be confused by the differences in their appearances.
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.
Maybe the Loud siblings could be transported to the TMNT universe. The boys would be fascinated by the Turtles' ninja skills and gadgets, and the sisters could use their own unique skills to help the Turtles in their fight against the Foot Clan. For example, Lola could use her pageant charm to distract the bad guys, while Lincoln could bond with the Turtles over being part of a big group.
The Turtles could interact with the Loud family by sharing their knowledge of martial arts. For instance, Raphael could teach Lincoln some self - defense moves. In return, the Louds could introduce the Turtles to normal family activities like having a family dinner or playing board games.
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.
Once upon a time, in a strange and wonderful world where the Teenage Mutant Ninja Turtles (TMNT) and the Equestria Girls (MLP EG) co - existed. Leonardo, the leader of the TMNT, met Twilight Sparkle from MLP EG at a magical portal. They were initially confused by each other's appearance but soon discovered their shared values of loyalty and friendship. As they spent more time together, they began to develop feelings for each other. Leonardo was drawn to Twilight's intelligence and magical abilities, while Twilight was intrigued by Leonardo's leadership and martial arts skills. Their love story became an inspiration for both their worlds, showing that love can cross the boundaries of different universes.
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.