A powerful demon could be the antagonist. They see the potential alliance between Gabriel and Loki as a threat to their plans for chaos and destruction. So they try to drive a wedge between them, using lies and illusions to make them turn against each other.
An ancient and powerful sorcerer from a forgotten realm. This sorcerer has his own agenda and fears that the combined power of Gabriel and Loki could disrupt his plans for domination. He uses his dark magic to create various obstacles for them, from summoning dangerous creatures to casting spells that cloud their judgment.
It could be another powerful deity from either the Norse or the Christian pantheon who doesn't want Gabriel and Loki to team up. Maybe a more rigid and traditional angel who thinks Gabriel is consorting with the wrong crowd by interacting with Loki.
One possibility is that they team up. They could find common ground in their love for causing a bit of chaos in a world that often seems too orderly. Together, they might play pranks on the other characters in their respective universes. For example, Loki could transform into a character from Supernatural and cause confusion among the hunters, while Gabriel uses his angelic mojo to mess with the Asgardians in a similar fashion.
Perhaps they meet during a cosmic event. Loki, always looking for chaos, gets involved in something that Gabriel is also a part of. They start off as adversaries but gradually realize that they can use each other's skills. For example, Loki's shape - shifting ability and Gabriel's powers of communication and persuasion could be combined to solve a greater threat to the universe.
Gabriel is an angel who has turned his back on the more serious aspects of his celestial duties in favor of a more human - like existence filled with humor and pranks. Loki, as a Norse god, has a certain level of pride and a thirst for power. Their personalities could clash when Loki's pride is wounded by Gabriel's nonchalant attitude towards power. However, they could also blend if they both realize that they can learn from each other. Gabriel could teach Loki to let go and enjoy life more, while Loki could show Gabriel that there are bigger things at play in the universe and that sometimes a little more strategy in his pranks could make them even more effective.
It could be someone who is manipulating Loki to kidnap the Hulk. Maybe an even more powerful villain from the Marvel universe, like Thanos. He might be using Loki as a pawn to get the Hulk for his own evil purposes.
It's hard to say which one is the sole main character. Gabriel has his own goals and plans which are central to the overall story of Miraculous. When he meets Plagg, his character development could take a new turn. On the other hand, Plagg is such a fun and unpredictable character. His reactions to Gabriel and how he might thwart Gabriel's plans could also make him a main focus. So in this fanfiction, they are both likely to share the role of main characters.
Probably Iron Man. Iron Man is always interested in new technology and knowledge, and Gabriel might have some otherworldly tech or knowledge to share.
In a 'gabriel is loki avengers fanfiction', the plot could start with Gabriel being mysteriously drawn to Loki's presence. He could be an angel - like being who is intrigued by Loki's mischievous nature. The Avengers, on the other hand, see Gabriel as a potential threat because of his association with Loki. As the story progresses, Gabriel could try to prove his good intentions to the Avengers while also trying to figure out Loki's true intentions. There could be a lot of internal struggle within Gabriel as he battles between his loyalty to Loki (if there is some pre - existing bond) and his desire to do what's right for the world. This could lead to a complex web of relationships and a story full of twists and turns as Gabriel tries to navigate the world of the Avengers and Loki.
Maybe an evil sorcerer who has mastered both dark magic and some forbidden ninja techniques. He plans to use the chaos that would ensue from merging the two worlds to his advantage. He would be against Naruto and Gabriel because they stand in the way of his evil plans. He could start by creating unrest in the Highschool DxD world, pitting different factions against each other, and then try to use Naruto's presence as a scapegoat. Gabriel, being an angel of justice, would naturally oppose him, and Naruto, with his sense of loyalty and protecting others, would join her in the fight.
An interdimensional being that wants to use Loki's unique connection to different realms. This being has the power to manipulate time and space, making it extremely difficult for the Avengers to reach Loki. The Avengers would need to figure out the being's weaknesses and use their own combined powers and skills to overcome it and rescue Loki.
Perhaps the main antagonist could be Loki himself, at least in the beginning. Since he got involved with Thanos' legs in some improper way, Thanos might see him as the antagonist. But as the story progresses, they might find a common enemy. So, initially, Loki's actions and motives that revolve around the legs could make him the antagonist in Thanos' eyes.