Well, for sure Harry Potter is a top pick. His connection to Gringotts started early on when he first visited to get his inheritance. Hermione would be there as she's the brains of the group and would be curious about any magical lore associated with Gringotts. Ron, with his family's financial struggles, could also play an important role. The goblin bankers are a given as they are the heart of Gringotts. In terms of crossover characters, if it was a crossover with 'Alice in Wonderland', the Cheshire Cat could make an appearance. It could be lured by the shiny objects in Gringotts and cause some chaos as it tries to steal something, with Harry and his friends having to stop it.
Obviously, Harry Potter would be the main character as it's in the title. But we might also see other familiar faces like Hermione and Ron. They could be helping Harry in his new role as the King of Gringotts. And of course, there would be goblin characters like Griphook who would play important roles as they are closely associated with the bank.
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.
Caleb Prior from Divergent and Ron Weasley might be a crossover. Caleb is intelligent but has his own moral dilemmas, and Ron is a loyal friend with his own insecurities. Their friendship or conflict could be explored in the fanfiction, perhaps with Caleb getting involved in Ron's wizarding adventures and Ron learning about the Divergent world through Caleb.
Obviously, Harry Potter himself would be a central character. He's the main protagonist in the series, so it's likely he'd be involved in the Christmas - related events. Hermione Granger would also be there as she's Harry's best friend and loves to participate in all things magical and festive. Ron Weasley too, as he's part of the trio. Dumbledore might also make an appearance, perhaps guiding the students in some Christmas - themed activities or sharing some wise words about the season.
For the Infamous side, Cole MacGrath would be a likely candidate. His powerful electrical abilities would make for an interesting dynamic in the Overwatch universe. On the Overwatch side, Tracer would be a great fit. Her speed and agility could contrast with Cole's more brute - force power. Also, Reinhardt could be involved. His chivalrous and protective nature would be interesting to see interacting with the Infamous characters.
Shepard from Mass Effect is a very likely character. Given their leadership skills and experience in dealing with various alien threats, they would be a natural fit in a crossover. They could potentially lead XCOM forces or at least be a key figure in coordinating between the two universes.
Definitely Harry Potter himself. He's the central character in his own series, so it's likely he'd be involved in any crossover. Hermione Granger could also be a key character as she's smart and curious, and would be interested in learning about the 'Twilight' world. From 'Twilight', Edward Cullen is an obvious choice. His brooding nature and unique abilities would make for interesting interactions with the 'Harry Potter' characters. Also, Jacob Black could be involved, especially when it comes to the 'imprint' aspect as he has his own story arc related to that in 'Twilight'.
Obviously, Harry Potter and Riddick would interact. Harry, being the main hero in his world, would be curious about Riddick's strange abilities and his rough - and - tumble way of life. Riddick, on the other hand, would likely be suspicious of Harry at first but then come to respect his magical powers.
Harry Potter and Ichigo Kurosaki are very likely to be paired. They are both the main protagonists in their respective series. Harry has his strong sense of justice and magical powers, and Ichigo has his determination and Soul Reaper abilities. They could form a great partnership to fight against common enemies.
Loki and Draco Malfoy would be a likely crossover. Loki is the mischievous god in the MCU, and Draco is a bit of a troublemaker in the Harry Potter world. They could team up and cause all sorts of chaos. Maybe Loki would teach Draco some of his more devious tricks, and Draco could introduce Loki to the pure - blood ideology of the wizarding world, which could lead to some really interesting storylines.