Buffy and Luke Skywalker could be an interesting pair. Buffy is a strong fighter and leader in her world, and Luke is a key figure in the Star Wars universe. They could learn from each other's combat styles and leadership skills.
Anya from Buffy and Han Solo might be a fun pairing. Anya has her own unique view of the world, often being quite practical about things like money. Han Solo also has that roguish, practical side. They could get into all sorts of adventures together, with Anya's knowledge of the supernatural and Han's piloting skills.
Darth Vader and Spock are also popularly paired in these crossovers. Vader's dark power and Spock's logical Vulcan nature create a great contrast. Fans like to explore how Spock might try to understand Vader's fall to the dark side, or how Vader would react to Spock's unemotional exterior. Princess Leia and Uhura are another pair that gets a lot of attention. Their strong female characters could collaborate on various missions and share their experiences of being in male - dominated fields in their respective universes.
Percy Jackson and Carter Kane would be a great pair. Percy is used to the Greek mythological world and its challenges, while Carter has a deep understanding of Egyptian magic and mythology. Their different perspectives could lead to some interesting teamwork, like when they face a monster that has aspects of both Greek and Egyptian mythology, such as a Sphinx with the ability to use water attacks like a Greek hydra.
Yoda and Alfred could also have an interaction. Yoda is a wise Jedi master, and Alfred is Batman's wise and loyal butler. They could exchange wisdom and perhaps Yoda could give Alfred some insights about the Force that Alfred could then use to help Batman in some way.
The Doom Slayer and Commander Shepard are likely to be paired. They are both iconic heroes in their respective universes. Shepard is a strategic leader, while the Doom Slayer is a force of pure destruction. Their interactions could lead to some really interesting storylines.
Harry Potter and Nate Ford could be a likely pair. Harry's experience with facing great evils and his leadership skills combined with Nate's strategic mind in planning heists would make for an interesting dynamic. They could learn from each other's methods of problem - solving.
Master Chief from Halo and Apollo from Battlestar Galactica could be an interesting pair. Master Chief is a super - soldier with incredible combat skills, and Apollo is a skilled pilot and military leader. Their different skillsets could lead to some great teamwork in a crossover story. Another possible pair could be Cortana and Starbuck. Cortana is an AI with vast knowledge, and Starbuck is a brash but talented pilot. They could have some really interesting exchanges, with Cortana providing strategic advice to Starbuck.
I think Buffy and Luke Skywalker would make an interesting pair. Luke has his Jedi training and the Force, and Buffy has her slayer strength and combat skills. They could learn a lot from each other. Luke could teach Buffy about the spiritual side of the Force, and Buffy could show Luke some of her street - smart fighting techniques.
Spike is often paired with Buffy in this type of fanfiction. Since they already had a complex relationship when Buffy was a slayer, it becomes even more interesting when she becomes a vampire. Angel is also a common pairing. Their past relationship and his own experience as a vampire with a soul add a lot of depth to the story.
The Joker could be a likely villain. Imagine his chaotic nature combined with the supernatural. He might team up with a demon to create even more mayhem in both Gotham and Sunnydale. His unpredictable actions would pose a huge challenge to Buffy and Batman.
Edward from Twilight could be a likely candidate. His brooding nature and special abilities might make for an interesting interaction with the Jedi or Sith in Star Wars. He could either be seen as a potential threat or an ally. Another would be Ahsoka Tano from Star Wars. Her strong will and combat skills could be interesting when pitted against the vampires of Twilight.