One idea could be that the Turtle Tots accidentally stumble upon a time - traveling device left behind by an enemy. They end up in the feudal Japan era. There, they meet young versions of their sensei and help him against some local villains. They learn about honor and the ways of the samurai, and when they finally return to their own time, they are more disciplined and better fighters.
In a time - travel story, the Turtle Tots might meet their future selves or versions of their family members from different times. These interactions can be crucial for character development. If they meet a future self who has become jaded or too aggressive, they can learn from that and decide not to go down that path. It can also show their growth if they are able to convince their future selves to change for the better.
Gohan could also time - travel to a point in Saiyan history. He might witness the destruction of Planet Vegeta and try to save some of the Saiyans. This would completely change the course of the Saiyan race and his own story. He'd have to deal with the cultural differences and the power - hungry nature of some of the Saiyans he saves.
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 Elrond travels back in time to the First Age to prevent a great tragedy. He uses his knowledge and wisdom from the Third Age to change the course of events. Maybe he stops a powerful weapon from being created that would later cause destruction.
For a travel - related private story, 'Journey's Hidden Gems' could be a great name. It implies that during your travels, you've discovered things that are special and not widely known, and these are part of your private story.
One idea could be that Finn is kidnapped by an old enemy who wants to use him as a bargaining chip against Princess Bubblegum. Maybe they demand her to give up the Candy Kingdom in exchange for Finn's safe return.
I really like 'The Time Traveling Lich'. In this fanfiction, the Lich uses time travel to try and rewrite history in his favor. It's full of action and has some really interesting twists on the characters' relationships. It shows how time travel could completely change the power dynamics in the Adventure Time universe.
One idea could be a modern-day historian who accidentally travels back to the Versailles negotiations after WW1. They could try to change the terms of the treaty, knowing the long - term consequences it had. For example, they might attempt to make the reparations more lenient on Germany, which could potentially prevent World War II.
A third idea is that Connor time - travels to a post - apocalyptic world. Here, he has to use his survival skills and assassin training to help the remaining survivors. He could also try to find a way to restore the world to its former state, all while facing new and dangerous threats.