Who could be the main antagonist in 'gabriel meets loki fanfiction'?

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.

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.

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.

Maybe a powerful warlord from Negan's world who somehow follows him into Artemis' realm. This warlord, with his own thirst for power and destruction, would oppose both Negan (if he starts to side with Artemis) and Artemis herself, wanting to take over the new world he has found.

It could be another god who is jealous of Zeus' interest in Atreus. Maybe a god who feels that Atreus is an outsider and doesn't belong in their divine affairs.

Another possibility is that Death is being controlled or influenced by a greater cosmic entity. In this case, that entity could be the main antagonist. It uses Death as a tool to test or destroy the Avengers. The Avengers would then have to not only deal with Death but also find a way to face this more powerful and mysterious entity behind Death's actions.

Kingpin could be a great antagonist. He has a long history with Daredevil and his presence in Hell's Kitchen could draw Peter in. His criminal empire and power would pose a significant threat to both heroes.

A powerful Decepticon could be the main antagonist. Maybe a Decepticon named Shockwave who is on a mission to find Bumblebee and extract some important information from him. He sees Dante as an annoying obstacle in his way and tries to eliminate him. Shockwave's advanced weaponry and cold, calculating nature would pose a great threat to Dante and Bumblebee's friendship and their goals.

Maybe an old family acquaintance of the Blacks who has a vendetta against Andromeda for leaving the family and is now worried that Draco, as a young Slytherin from a prominent family, is being led astray by her. This character could use their knowledge of the Black family secrets and connections to create obstacles for Draco and Andromeda as they try to build a relationship.

It could be a dark wizard who wants to capture Nagini for his own evil purposes. Maybe he wants to use Nagini's connection to Voldemort to gain power.