Definitely Naruto and Sasuke. In a 'past Naruto meets Boruto' fanfiction, if Sasuke was also involved in the meeting, it would be really cool. Boruto could see the dynamic between his father and Sasuke, two of the most powerful ninjas. He could learn about their past rivalries and how they overcame them to become allies. It would be a great opportunity for Boruto to understand the importance of having a strong bond with your comrades.
The interaction between Naruto and Kakashi could be very interesting in this fanfiction. Kakashi was Naruto's mentor, and he has a wealth of knowledge. Boruto could witness their relationship first - hand. Naruto could ask Kakashi for advice on how to be a better father to Boruto, while Kakashi could share some old - time ninja wisdom with Boruto. It would be a great way to bridge the gap between the past and the present in the Naruto universe.
Definitely Doctor Strange and Loki. Loki is a master of trickery and Doctor Strange is a master of magic. Their interactions would be full of mind games and magic duels. Loki might try to deceive Strange, but Strange would see through his illusions with his magic.
The interaction between Starfire and Beast Boy would be fascinating. Beast Boy can transform into different animals, and Starfire has her own set of superpowers. They could have some really fun and comical moments together, like when Beast Boy turns into a small animal and Starfire is trying to figure out how to communicate with him. Also, they could team up in battles in really creative ways, with Beast Boy using his animal forms to distract enemies while Starfire blasts them with her energy.
The interaction between Robin and Batman would be really interesting. Robin has his own identity with the Teen Titans now, and Batman is his former mentor. There would be a lot of unspoken history and emotions. Batman might be proud of how Robin has grown, but also a bit overprotective. And Robin would be eager to show Batman that he can handle himself without his constant guidance.
I think Kakashi's interaction with Minato would be very interesting. Kakashi was Minato's student. Meeting his sensei again, especially with Team 7 in tow, would bring back a lot of memories. Kakashi could show Minato how he has grown and passed on Minato's teachings. And Minato could give Kakashi some new insights based on his own experiences.
Rin and Snape would also make an interesting pair. Snape is this brooding and complex character. Rin might initially be put off by his cold exterior but as she gets to know him, she could understand his pain and the reasons behind his actions. And Snape would be curious about Rin's magic which is so different from what he's used to. They could have some tense but also very thought - provoking exchanges.
Definitely Garfield and Leon. Leon is such a strong and respected trainer in Pokemon Sword and Shield. Garfield, on the other hand, is this lazy cat. The contrast between their personalities would make for an interesting interaction. Leon might be really confused by Garfield's lack of interest in becoming a great trainer and just his focus on food.
I think the interaction between Red Robin and Batman would be the most interesting. Since Red Robin is Batman's protege, seeing how their relationship plays out in the context of the Justice League would be cool. Maybe Batman would be more protective or more critical of Red Robin in front of the other League members.
I think the interaction between Sonic X's Knuckles and Sonic Boom's Knuckles would be really interesting. They both have their own sense of duty and pride, but their approaches might be different. Sonic X Knuckles is more traditional in his guarding of the Master Emerald, while Sonic Boom Knuckles might be a bit more laid - back. It would be fun to see them clash and then come to an understanding.
It would probably be the Shinigami. Since it's a mysterious and powerful being from another realm, its motives and actions would be really interesting to follow.
I think Anzu and Sakura would make an interesting pair. They are both strong - willed female characters. Anzu could introduce Sakura to the world of Duel Monsters, and Sakura could teach Anzu some basic ninja self - defense techniques. They could bond over their shared experiences of being friends with the main male characters in their respective series and also support each other in their own personal growth.