One interesting crossover could be the use of magic in the Star Trek universe. For example, a fanfiction might have a wizard or witch from Harry Potter end up on the Enterprise and use magic to solve some of the ship's problems, like a strange alien energy field that normal technology can't deal with.
In a fanfiction of this kind, we might see the Hogwarts houses incorporated into Star Trek. The Sorting Hat could be brought into the Star Trek universe and sort the crew members. Spock might be a Ravenclaw for his intelligence, while Kirk could be a Gryffindor. Also, the idea of the Deathly Hallows in the Star Trek world is fascinating. The Elder Wand could be a powerful weapon against the Klingons or other enemies.
There might be crossovers where the characters interact. Picture Captain Kirk meeting Harry Potter. Kirk could introduce the concept of exploring new galaxies to Harry, while Harry could show Kirk some magic spells. In fanfiction, this could lead to all sorts of adventures, like the two of them teaming up to fight a common enemy that threatens both the Federation and the wizarding world.
One interesting crossover could be the use of magic in the Star Wars universe. For example, a Harry Potter character might introduce spells to the Jedi or Sith. It could add a new dimension to lightsaber battles, like using a spell to disrupt the energy of a lightsaber. Another aspect could be the cultural exchange. The Hogwarts houses could be compared to the different factions in Star Wars. Gryffindors might be seen as similar to the Rebel Alliance, with their courage and sense of justice. Slytherins could have parallels to the Sith in terms of their ambition and cunning.
There could be a fanfiction where the characters from both universes meet at a sort of interdimensional crossroads. For example, Hermione could team up with Spock. Hermione's vast knowledge and logical thinking, combined with Spock's Vulcan logic, could make for an interesting duo as they try to find a way back to their respective universes while also facing various challenges along the way.
One interesting crossover could be Kim Possible characters getting involved in a Star Trek mission. For example, Kim could use her martial arts skills to help the Star Trek crew in a physical confrontation on an alien planet. Maybe Ron Stoppable's clumsiness could lead to some comical yet ultimately helpful situations, like accidentally activating a crucial device on the Star Trek ship.
A common crossover is the appearance of Starfleet officers in the Warhammer 40k universe. They might be seen as strange and otherworldly beings. Their peaceful exploration ethos would be a sharp contrast to the constant warfare in 40k. For example, they could try to broker peace between warring factions in 40k, which would be a huge challenge given the nature of that universe.
One interesting crossover could be the sharing of technology. For example, the warp drive from Star Trek might be compared or combined with the hyperspace technology in Babylon 5. Another aspect could be the meeting of different alien species from both universes. Maybe the Vulcans from Star Trek and the Minbari from Babylon 5 could engage in some sort of philosophical exchange.
Well, in some fanfictions, you might see the Death Star being infiltrated by Hogwarts students using their magic skills. They could use invisibility cloaks to move around undetected and use spells to disable the security systems. Also, the idea of a lightsaber duel between a Star Wars character and a Harry Potter character who uses a magically enhanced sword could be really cool. It would be a clash of two very different but equally powerful combat styles.
One interesting crossover could be the use of magic in the Star Wars universe. For example, a wizard from Harry Potter might visit the Star Wars galaxy and use spells to interact with the Force. Another could be the characters from both universes teaming up. Maybe Harry Potter and Luke Skywalker could join forces to fight a common enemy that threatens both their worlds.
Well, in some Star Trek vs Mass Effect fanfiction, there are often epic battles. Imagine the Borg from Star Trek facing off against the Reapers from Mass Effect. It would be a battle of assimilation vs destruction on a galactic scale. Also, the Starfleet officers' principles and ethics could clash or blend with the Spectres' more flexible code of conduct in Mass Effect.