Thanos is a likely one. Given their both having grand ambitions in the Marvel universe. In fanfiction, their meeting could be a battle of wills as they both want to be the dominant force. Loki might initially try to deceive Thanos, but Thanos is no fool and would see through Loki's tricks.
Doctor Doom could also be one. He is a powerful and intelligent villain. If Loki meets him, they could have an exchange of knowledge. Doom might be interested in Loki's magic, and Loki could be intrigued by Doom's scientific and magical hybrids. Their encounter could be filled with a lot of posturing and attempts to one - up each other.
I think it could be Star - Lord. He's often looking for an edge in any situation, and Loki could offer some unique opportunities. They could form a sort of uneasy alliance where they both try to use each other for their own goals. Star - Lord might see Loki as a valuable asset in terms of his magic and knowledge of other worlds, while Loki could use Star - Lord's leadership skills and connections within the Guardians. However, they would constantly be on guard against each other's double - crosses.
It could be Danzo. He's always been a shadowy figure with his own agenda in the Naruto series. In a massacre fanfiction, he might be behind it all to gain more control over the Hidden Leaf Village. He has no qualms about sacrificing others for his goals. Also, Kabuto could be a villain as he is a follower of Orochimaru and has his own motives for causing chaos.
Iron Man and Batman might be the most likely to clash. Iron Man is all about high - tech and showing off his latest inventions. Batman, while also using technology, is more of a brooding, solitary figure. Iron Man might see Batman as a bit of a relic or too old - fashioned in his methods. Batman, in turn, could view Iron Man as a bit too flashy and irresponsible with his power. Their egos could get in the way of them working together initially.
Annihilus could be a likely villain. He is a powerful being from the Negative Zone. In the fanfiction, Annihilus could be drawn to Earth because of the energy signature created when the Avengers and Captain Marvel meet. He sees Earth as a prime target for conquest. His powerful cosmic control rod gives him an edge in battle. The Avengers and Captain Marvel have to quickly learn about his weaknesses and use their combined powers to stop him from destroying Earth and spreading his reign of terror across the universe.
Dumbledore could be a very likely ally for Loki. Dumbledore is known for his wisdom and ability to see the good in people. He would recognize Loki's power and his potential for both good and bad. Dumbledore could guide Loki in understanding the values of the wizarding world and in return, Loki could offer his unique skills and knowledge. Dumbledore would also be able to keep Loki in check if he were to get too carried away with his mischief, and together they could face threats to the wizarding world that require a combination of Asgardian and wizard magic.
Doc Ock is a likely villain in Spider - Man angry fanfiction. His mechanical arms and his intelligence make him a formidable foe. His evil plans usually put the city in danger, and Spider - Man would be extremely angry trying to stop him. Also, Electro could be a villain in such fanfictions. His ability to control electricity can cause a lot of destruction, and Spider - Man would be angry at his disregard for human life while causing chaos.
The Joker could be a likely villain. Imagine his chaotic nature combined with the supernatural. He might team up with a demon to create even more mayhem in both Gotham and Sunnydale. His unpredictable actions would pose a huge challenge to Buffy and Batman.
Thor would be a likely candidate. Since he is Loki's brother, he already has a connection with him. Thor might be the one to convince the other Avengers to give Loki a chance and be the first to reach out and try to befriend him in this new situation.
Doctor Strange and Ajak. Doctor Strange is well - versed in the mystic arts and Ajak has a lot of knowledge about the Eternals' history and their connection to the Celestials. Their knowledge areas could intersect, and they would likely be the first to start sharing information. Maybe Strange senses some strange energy related to the Eternals and approaches Ajak to find out more.
Thor would be the most likely. Since Thor has a connection to Norse mythology already, he would be familiar with Sleipnir. He might see Sleipnir as a symbol of his home realm and would quickly bond with it. They could communicate in a way that the other Avengers can't, sharing stories of Asgard and the great battles there.