Definitely Damon Salvatore. He's the bad - boy vampire with a heart of gold (sometimes). In a crossover, he'd be a wild card. He has his own sense of justice and loyalty, but also a devil - may - care attitude. He could get into all sorts of trouble in the world of the Originals. His sarcastic remarks and his ability to charm his way out of (or into) situations would make for some great scenes. He might also form an unlikely friendship or rivalry with one of the Originals, like Kol perhaps. His backstory and his experiences in 'The Vampire Diaries' would contrast and collide in interesting ways with the Originals' world.
I think Caroline Forbes could be an interesting character to focus on. She's from 'The Vampire Diaries' which is related to 'The Originals'. If she were to be part of a 'The Originals' and 'Supernatural' crossover, she could bridge the gap between the two worlds. She's a strong - willed vampire who has learned to adapt and grow. Her positive attitude could contrast with the more brooding nature of some of the 'Supernatural' characters, and she could bring a fresh perspective to the team - up against whatever supernatural threat they face.
I think Rose and Klaus would be an interesting pairing. Rose is a tough and feisty guardian from Vampire Academy, and Klaus is a powerful and complex original vampire. Their interactions would be full of sparks, as they both have strong personalities and a lot of power. They could either be at odds with each other or end up having an unexpected alliance.
Klaus Mikaelson would be a really interesting character to crossover. He has such a complex personality - part villain, part anti - hero. In The Vampire Diaries, he was a major threat, but in The Originals, we see more of his vulnerable side. Bringing him back to Mystic Falls in a crossover would create a lot of drama. His power and his history with the characters there would make for some great storylines.
Willow from Buffy's world and Davina from the Originals could make for an exciting pairing. Willow is a powerful witch, and Davina is also very talented in magic. They could exchange spells and knowledge, and maybe even create new magic together to fight a common enemy.
Klaus and Merlin would be a very interesting pairing. Klaus is a complex and often villainous character in the Originals, while Merlin is a hero in his own story. Their clash of personalities and power dynamics would be fascinating. Merlin's moral compass could be a challenge to Klaus' more self - serving ways.
Raphael could be really interesting. In the 2003 version, he is hot - tempered and tough. In the 2012 version, he also has a strong personality. Seeing how he reacts to his counterpart and how they might either clash or bond could be a great focus. Maybe they have a friendly rivalry as they compare their fighting skills.
Ross could be really interesting. Since he's an archaeologist, in a crossover with a show like 'Indiana Jones' (in a fanfiction sense), he could go on an adventure with Indiana Jones. Ross would bring his academic knowledge while Indy would have the practical experience. Their interactions would be filled with Ross trying to over - analyze everything and Indy just wanting to get the job done.
Chiaki Nanami could be an excellent choice. She is the Ultimate Gamer, so if you cross her with a digital or virtual world, there are so many possibilities. She could use her gaming skills to navigate through a new digital realm, like a Matrix - like world. Her calm and intelligent nature would also make her a great character to lead the group in a crossover situation. She could figure out the rules of the new world quickly and help the others adapt.
Arya Stark and Rebekah Mikaelson. Arya is a trained assassin, and Rebekah is a fierce vampire. They could learn a lot from each other. Arya could teach Rebekah some of her stealth and combat skills without the use of vampire powers, and Rebekah could introduce Arya to the world of the supernatural and its different rules of survival.
It's likely Klaus Mikaelson. He's an Original vampire - werewolf hybrid in The Originals. He has super strength, speed, and his werewolf side gives him an edge in combat. In comparison, the vampires in Twilight, while strong, don't have the added advantage of being part werewolf. Also, Klaus has a lot of experience and is very cunning, which would make him a formidable force in a crossover situation.