Definitely Torbjorn. Since he has a connection with Bastion in the original Overwatch lore, he would be a central character. Reinhardt could also be involved. His chivalrous nature might lead him to protect Bastion if he saw it was in trouble. And of course, Mercy. She's always there to heal and help, so if Bastion gets damaged during the story, she would be there to patch it up.
Well, Bastion being the main focus, Torbjorn is very likely to be in the story as he was involved in Bastion's creation. Pharah could also be part of it. Her aerial abilities would be useful in scouting and providing cover during battles. Soldier: 76 might be there too. His combat experience and leadership could be crucial in guiding the team when they are facing threats related to Bastion's situation. There could also be an appearance from Zenyatta, who might offer some spiritual or philosophical insights to Bastion or the other characters in the story.
One possible plot for this fanfiction could be that Bastion accidentally activates an ancient Omnic protocol. This protocol sends out a signal that attracts the attention of both Overwatch and a group of evil Omnics. The Overwatch heroes, including Bastion's friends like Torbjorn, come to Bastion's aid. Together, they have to fight off the evil Omnics and find a way to deactivate the protocol before it causes more chaos. It could be a story of friendship, sacrifice, and the struggle between good and evil in the Overwatch universe.
Hermione Granger is also a likely character. Her intelligence and love for knowledge could lead her to discover a way to bridge the past and the future. She might use her Time - Turner knowledge or find some new magical method. And she could play a key role in helping past characters adapt to the future or vice versa.
Probably Naruto and Tracer. Naruto is always eager to meet new people and make friends, and Tracer's curious and friendly nature would make her approach Naruto. They could start by comparing their abilities. Naruto would show off his Rasengan while Tracer might do a quick display of her blinking ability.
Sasuke is very likely to be involved. He has a close relationship with Naruto and is also an important figure in Boruto's life. His skills and knowledge could be crucial in any adventure or situation that Naruto and Boruto find themselves in. Another character could be Hinata. As Naruto's wife, she would surely be concerned about any strange events involving Naruto. And for Boruto, she is his mother, so she would play a role in any family - related or village - threatening situations.
Homer Simpson is very likely to be involved. He's the main character and his bumbling yet loveable nature can create a lot of comical situations in crossovers. For instance, if it's a crossover with 'The Flintstones', Homer would probably be trying to out - eat Fred Flintstone at the local diner.
Sasuke is very likely to be involved. Since he's Naruto's rival and friend, he might be there to witness the meeting of different Narutos. He could be confused at first, but then he'd probably try to figure out how this strange phenomenon occurred. He might also get involved in battles or discussions that the Narutos are having.
Obviously, Harry Potter himself could be involved. He might be a mentor figure in the future, passing on his knowledge and experiences to the new generation. His children, if the fanfiction extends to that, would also be key characters. Hermione and Ron could also play important roles, perhaps as educators or as important figures in the wizarding government. Neville Longbottom could be involved too, maybe as a herbology expert who is helping to develop new magical plants for the future wizarding world.
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.
Theon Greyjoy is also a candidate. His identity and self - worth are often tied to his appearance. A headshave in fanfiction could be a way to explore his further degradation or, conversely, his redemption as he sheds his old self - image and tries to start anew.
Shinji is a very likely character. His complex relationship with his father and his role as the main pilot make him a prime candidate for time travel scenarios. He could go back in time to try and fix things or understand his father better.