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.
Maybe the Turtles from both universes team up to fight a common enemy that is a threat to both their worlds. They could combine their unique skills and weapons. The 2003 Turtles' more serious and tactical approach could blend well with the 2012 Turtles' more energetic and modern fighting styles.
A great plot could be that an old enemy from the 2003 universe escapes into the 2012 universe and the Turtles from both versions have to work together to stop it. Along the way, they learn about each other's strengths and weaknesses. The 2003 Turtles might be more disciplined in their training, while the 2012 Turtles could introduce new technology to their counterparts.
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 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 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.
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 possible plot could be that the Teenage Mutant Ninja Turtles somehow end up in the Freddy Fazbear's Pizza place. Maybe they are investigating some strange disappearances that are rumored to be related to the animatronics. They could use their ninja skills to navigate the dark hallways and face off against the haunted animatronics. The turtles' unique personalities would add a fun dynamic, like Mikey being scared at first but then finding the whole situation exciting, while Raph is ready to fight from the start.
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.
Perhaps the mutagen that created the Turtles has some strange reaction when exposed to the Dragon Balls' energy. This could lead to new powers for the Turtles and a whole adventure about controlling and understanding these powers while also dealing with the Dragon Ball Z characters' reactions to them. For example, Vegeta might be initially suspicious and see the Turtles as a threat to his power status.
Perhaps a plot where Leo discovers some new powers or abilities that are related to his pregnancy. It could be something that he has to learn to control quickly. And then there could be a story arc about the team finding a way to ensure a safe delivery for Leo, which might involve seeking out some very rare or magical items.