Who could be the main villain in 'Steve gets killed' Avengers fanfiction?

One possible plotline could be how the other Avengers react to Steve's death. Some might go into deep mourning while others could be driven by revenge. For example, Tony might use his technology to try and find out who was responsible and bring them to justice.

Thanos could also be a good option. Imagine Bella's planet holds one of the Infinity Stones that Thanos needs. He would stop at nothing to get it, and Bella and the Avengers would have to join forces to protect it. Thanos' brute strength and strategic mind would pose a huge challenge for them.

A powerful necromancer could be a great main villain. He tries to control the ghostly avengers for his own evil purposes, using dark magic to manipulate them.

It could be Loki. He might have some new trick up his sleeve where he makes the Avengers forget Tony as part of a bigger scheme to cause chaos among the team and weaken their defenses. He could be planning to take over the world while the Avengers are distracted by this strange memory loss.

Maybe it could be a time - traveling criminal. This villain has the ability to jump between different timelines and has noticed that by disrupting the key events in both the Arrow and Avengers universes, he can create chaos and then rule over the resulting mess. He would constantly be one step ahead of the heroes as he could manipulate time to his advantage. He could also use his knowledge of future events to set traps for the Avengers and the Arrow team.

It could be an evil scientist. This scientist was conducting experiments to create superhumans, but his experiments went wrong. Now, he's jealous of Marco's natural - born powers. So, he tries to capture Marco and study him to find a way to replicate his powers for himself. The scientist creates all kinds of dangerous traps and robotic minions to try and catch Marco.

Dr. Eggman would be a likely main villain. He might be the one who devises a plan to make Sonic fat, thinking that it will slow him down and make it easier for him to be defeated.

It could be a long - lost relative of an old enemy. For example, if there was a powerful dark wizard that Natsu defeated in the past, this relative might come seeking revenge and be powerful enough to kill Natsu.

Maybe there's a young tech - savvy kid. The kid could be the one who shows Steve all the cool gadgets like virtual reality headsets. They become friends and the kid helps Steve navigate the digital world, while Steve in turn teaches the kid about things like loyalty and honor from his own time.

Maybe in a fantasy fanfiction, an evil sorcerer is the villain. Christy might be a key figure in a prophecy to save the kingdom. So, the sorcerer tries to stop her by casting a spell that makes her get hurt. He doesn't want her to fulfill her destiny and will do whatever it takes to keep her from being a threat to his power.