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 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.
One idea could be that the Turtles end up in the Underground world of Undertale. They might have to navigate through the various regions, like Snowdin or Hotland, using their ninja skills. For example, they could use their stealth to avoid some of the more dangerous monsters. Another plot could involve the characters from Undertale being brought to the Turtles' New York. They would then have to deal with the Foot Clan and other threats in this new, strange environment.
One idea could be that the Turtles and the Power Rangers team up to fight a common enemy that has the ability to morph between dimensions. Another could be a story where the mutagen from the Turtles' world gets mixed with the Power Rangers' technology, creating new and unexpected powers for both groups. Or perhaps a plot where the villains from both universes join forces, forcing the heroes to overcome their differences and work together.
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.
Well, a really interesting plot might involve the Agents of Shield trying to recruit the TMNT for a special mission. Since the turtles have unique skills and knowledge of the sewers and the underbelly of New York, they could be valuable assets. Or perhaps a Kraang invasion occurs in the Agents of Shield universe, and the Agents have to work with the turtles to stop the alien menace. The turtles could teach the Agents some of their ninja fighting techniques while the Agents share their high - tech weaponry and strategies.
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.
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.