Han Solo and a Skaven from Warhammer. Han's roguish charm and ability to get out of tough situations could be pitted against the devious and sneaky Skaven. The Skaven's underhanded tactics and Han's quick thinking would make for some really entertaining encounters as Han tries to outwit the Skaven in various situations across the two universes.
Definitely Indiana Jones and Han Solo. Indiana's tenacity and knowledge of archaeology combined with Han's roguish charm and piloting skills would make for some great interactions. They could be on a quest for a valuable artifact that has significance in both the Star Wars galaxy and on Earth.
Definitely Ben Tennyson himself. His ability to transform into various aliens would add a whole new dynamic to the Star Wars universe. Also, characters like Darth Vader would be great. Imagine the confrontation between Ben in one of his powerful alien forms and Vader with his lightsaber and the Force. Another interesting character could be Yoda. Yoda's wisdom and his unique view of the Force could interact in an interesting way with Ben's more action - based alien powers.
Definitely Ash Ketchum. He has so much experience with Pokemon and would be really out of place but also very interesting in the Star Wars universe. He could learn about the Force and maybe use it to communicate better with his Pokemon.
Commander Shepard would be a great character to include. His/her leadership skills and combat experience could be put to the test in the brutal and unforgiving Warhammer 40k universe. Maybe facing off against a Chaos warband or trying to negotiate with an Inquisitor.
Definitely Master Chief and Luke Skywalker. Master Chief is a super - soldier with incredible combat skills and years of military experience. Luke Skywalker is a powerful Jedi Knight. The contrast between their powers and the way they approach battles would be fascinating. Luke could teach Master Chief about the Force, and Master Chief could show Luke some of the high - tech combat strategies from the Halo universe.
Pairing the Witch Queen from Destiny with Darth Vader would be an intense combination in a 'Destiny Star Wars crossover fanfiction'. The Witch Queen's powerful magic and her manipulative nature against Darth Vader's dark side powers and his loyalty to the Emperor. It would be a battle of wills and a struggle for power as they either try to outwit each other or find some common ground.
Thor and a Dwarf Lord would be really interesting. Thor with his god - like powers and his hammer Mjolnir could team up with a Dwarf Lord, who is known for his skill in forging and combat. They could exchange stories about their great battles and also work together to create some powerful weapon, perhaps using Asgardian and Dwarven forging techniques.
Anthony Bridgerton would be a great character to include. His strong sense of family and his often stubborn nature could lead to some interesting clashes in a crossover. Say, in a crossover with a fairytale world like Cinderella. He might initially be against the idea of the Bridgerton family getting involved with the magic and 'foolishness' of the fairytale world, but then gradually change his mind.
Megatron and Darth Vader could be a very dark and powerful pairing. Both are iconic villains. Megatron's thirst for power over Cybertron and Vader's loyalty to the Emperor are similar in nature. They could form an alliance, plotting to take over both the Cybertronian and Star Wars worlds. Their combined might would be a huge threat to the heroes, and it would be interesting to see how the Autobots and the Jedi would counter them.
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.