Who could be the main antagonist in an 'Assembled we are strong' Avengers fanfiction?

It could be about the Avengers coming together after a major setback. Maybe they've lost a big battle against a powerful new villain. But then they realize that by truly assembling as a team, sharing their powers and strategies, they become strong enough to take on the foe. For example, Iron Man could combine his technology with Thor's magic in a way they hadn't thought of before.

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.

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.

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.

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.

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.

Voldemort could be a likely main antagonist. He's the most powerful dark wizard in the Harry Potter series. He might see the Avengers as a threat to his plans of total domination, not understanding their world but still seeing their power. So, he could use his dark magic to torture them, trying to break their will before eliminating them.