Well, you could have a plot where the technology of Earth X is the key to solving a major problem in the multiverse. The heroes from different Earths have to work together to obtain and utilize this technology. However, there are also those on Earth X who don't want their technology to be used by outsiders. This creates a conflict within the crossover story. The heroes not only have to deal with external threats but also internal disputes on Earth X.
A possible plot is centered around the relationships that form during the crossover. Imagine a love story between two characters from different Earths. Maybe a hero from Earth X falls in love with a hero from the main Earth. This relationship could be complicated by the differences in their worlds and the events of the Crisis. Also, a plot about the heroes having to adapt to the different rules and cultures of Earth X would be engaging.
One idea could be that the Ranma characters get transported to the Bubblegum Crisis world. Ranma could be confused by the high - tech setting and the existence of Boomers. Maybe he would initially try to use his martial arts against them, thinking they are just some strange opponents. Another plot could involve the Knight Sabers somehow getting involved in a martial arts tournament in the Ranma world. They would have to adapt to the different fighting styles and rules there.
One idea could be Jedi coming to Earth to train a select group of humans with Force - like abilities. Another could be a Sith Lord arriving on Earth and causing chaos, and Earth's heroes have to team up with Star Wars characters to stop him. Maybe there's a discovery of a hidden Star Wars - related artifact on Earth that has the power to change the balance of the Force.
A great plot idea would be a crossover with 'Game of Thrones'. The political intrigue in both worlds could be merged. Maybe Talion could find himself in the middle of the power struggle between the different houses in Westeros. He could use his skills to either side with a particular house or try to stay neutral while still achieving his own goals in Middle Earth which are now somehow intertwined with the events in Westeros.
A crossover with a modern - day setting could also be really cool. Picture Talion and Celebrimbor suddenly appearing in a big city. They would have to adapt to modern technology while also dealing with an evil force that has followed them through some sort of magical portal. The contrast between their medieval - style combat skills and the high - tech environment would make for some really interesting scenarios.
One idea could be that Godzilla Earth teams up with a group of humans who have developed advanced technology to fight off an alien invasion. The humans initially fear Godzilla but soon realize they need his power.
A crossover with Stargate SG - 1 could also be fascinating. The Goa'uld and the Taelons both have an interest in power and domination. The SGC could get involved when they notice the Taelons' activities on Earth. They might try to figure out if the Taelons are another alien race like the Goa'uld or something entirely different.
One idea could be a character from the Enclave finding themselves in a situation where they have to team up with a group from the Fallout world they usually oppose. Maybe a lone Enclave soldier gets separated from his unit and is forced to rely on a group of wastelanders to survive. This could lead to a lot of character development as he sees a different side of the world outside the Enclave's ideology.
Well, in some fanfictions, there might be a plot twist where the portals between the different Earths malfunction in a new way. Instead of just allowing the heroes and villains to cross over, they start merging certain parts of the different Earths. So you might have a city on Earth - 1 that suddenly has a section that looks like it's from Earth - X. This would create all sorts of chaos and new challenges for the characters to deal with. It also gives an opportunity for different versions of the characters to interact in unexpected ways, like a more peaceful version of a villain from one Earth having to face off against their more malevolent counterpart from another Earth that has now invaded their space.
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 a crossover with Undertale. The characters from Glitchtale could interact with the original Undertale characters in a new and unexpected way. For example, Betty from Glitchtale could team up with Chara from Undertale for some sort of dark plan. Another plot could involve a crossover with a different game like Minecraft. The Glitchtale characters could find themselves in a blocky world, having to adapt to new rules and enemies. Maybe Sans from Glitchtale could use his powers in the Minecraft world in a really creative way.