It could be one of the Avengers themselves. Maybe Iron Man, if he feels that T'Challa's technology in Wakanda is a threat to his own tech empire. He might start some sort of power play which makes the Avengers not friendly towards T'Challa.
Another possibility is an outside force that manipulates the Avengers into believing T'Challa is a threat. For example, a villain who wants to create chaos between Wakanda and the Avengers. They could plant false evidence or spread rumors, turning the Avengers against T'Challa.
Maybe it could be about the Avengers having a misunderstanding with T'Challa. For example, they might misinterpret his actions in protecting Wakanda as being too self - centered and not in line with the Avengers' more global view.
Loki could be a great main antagonist in this context. He's always been one to cause chaos and undermine the Avengers. He could manipulate events to make it seem like the Avengers are causing injustice, perhaps by framing them for crimes they didn't commit. His mischievous nature and magical abilities would make him a formidable foe.
A new character, like a powerful interdimensional sorcerer who wants to merge all the universes into one chaotic realm under his control. He has the ability to manipulate the fabric of the multiverse and can summon different versions of villains from various universes to do his bidding. The Avengers would have to not only fight his minions but also figure out a way to stop him from achieving his goal of multiversal domination.
One great aspect of fanfiction involving Tony and T'Challa could be their different technological approaches coming together. Tony is all about high - tech, flashy Iron Man suits and advanced weaponry. T'Challa, on the other hand, has the advanced Wakandan technology which is more rooted in tradition and secrecy. In a fanfiction, they could collaborate on a project to upgrade the Avengers' technology while also respecting Wakandan traditions. For example, they might combine Tony's miniaturization techniques with Wakandan vibranium - based energy sources to create a new kind of power cell for the Avengers' equipment.
A new, original villain could also be the main antagonist. This villain could be someone from Percy's past who has a grudge against him and wants to use the Avengers as a way to get revenge. By making Percy betray the Avengers, this villain can not only harm Percy but also cause chaos among the powerful superhero group.
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.
It might be an alien race that has the ability to travel through the earth's crust undetected. They are stealing valuable resources from the planet's core. The Avengers, including Quake, are the only ones who can stop them. The aliens' technology and physical abilities make them a formidable foe, and Quake has to use her powers in creative ways to expose and fight them.
Maybe an old enemy from Wakanda's past. This enemy sees the deaged Tony and T'Challa as easy targets. He wants to steal Wakanda's technology and Iron Man's tech. He tries to manipulate the deaged heroes and use them for his own evil plans.
A technologically advanced alien race could be the main antagonist. They have studied the Avengers and know their weaknesses. They attack Earth in a way that isolates each Avenger at first. But when the Avengers assemble, they use their combined strength to hack into the alien technology, reverse - engineer it, and turn it against the aliens.
Loki could be a great main antagonist. He might see Nico Di Angelo as a new plaything or a threat to his plans. Since Nico has powers related to the underworld, Loki could try to manipulate him or use his powers for his own evil schemes, which would put him in direct conflict with the Avengers.