Who could be the main antagonist in a 'captain rex dies fanfic'?

Maybe a group of bounty hunters hired by the remaining Imperial forces. They see Luke as a big threat and Captain Rex as an annoyance. They keep hunting them down throughout the story, creating all sorts of dangerous situations for our heroes.

A rogue ninja from another village could be the main antagonist. Maybe a ninja from the Hidden Mist Village who has a personal grudge against Konoha and sees Naruto as the symbol of Konoha's strength. So, he devises a complex plan to take down Naruto. This ninja could have some unique and deadly jutsu that Naruto has never faced before, which ultimately leads to his downfall.

It could be a powerful sorcerer who wants to take over Crookshnks' homeland. This sorcerer has dark magic and sees Crookshnks as a threat to his plans, so he devises a scheme to eliminate him.

A new organization that is bent on world domination could be the main antagonist. They set a trap during Naruto's mission. This organization might have access to some ancient and forbidden jutsu that they use against Naruto. They see Naruto as a major threat to their plans, so they target him specifically.

It could be a rogue ninja who was secretly plotting against Naruto all along. After Naruto's death, this antagonist could emerge and try to take over the village, seeing it as the perfect opportunity with the village in mourning and a bit disorganized. This ninja might have some personal vendetta against Naruto and now wants to make the village pay.

It could be a rival family who had some sort of long - standing feud with Sakura's family. They might have caused their deaths in an underhanded way and now Sakura has to deal with them as she tries to uncover the truth.

It could be an ancient dark wizard who has been resurrected and sees Callum Hunt as a threat to his plans of world domination. This wizard could use his powerful dark magic to overpower Callum and ultimately cause his death.

The Night King could be a great antagonist. His army of the dead and his own powerful magic would pose a huge threat to Captain America. Cap's used to fighting human foes, but the Night King is something entirely different.

In some possible scenarios, he might die heroically in a battle against a powerful enemy. Maybe while defending his comrades or an important location.

Thanos could still be a great antagonist even in a fanfic. He could come back with a new plan to obtain the Infinity Stones again. His powerful and menacing presence would create a high - stakes situation for Captain Marvel and the Avengers to face.