One of the main draws of anime lies in the genre's ability to flesh out even the most static of characters into compelling personalities. Because it its rich storytelling, the genre produces plenty of anime villains who deserve their own anime. All sorts of sympathetic anime antagonists exist, giving studios tons of content to work off of. These villains offer a new perspective on the original content, one that is otherwise impossible through the protagonist alone.
Scar of Fullmetal Alchemist presents a window into Ishvalan society you never see in the primary storyline. On the flipside, a villain like Hunter X Hunter's Hisoka is so fascinating you would watch 150+ episodes worth of him stuck at the DMV just for more screen time. Of all the anime characters deserving of their own series, the most pressing ones might just be the anime villains. Sometimes, bad people give you the best content.
Naruto boasts an impressive cast of worthy villains, but the one with the greatest spin-off potential is definitely Madara Uchiha. With a ninja career that began at age five, Madara grew up as a member of a heavily persecuted family. Due to his rough upbringing, he sees the world as inherently corrupt and therefore wants to destroy it to make room for something better.
Madara's story not only would give insight into every other member of the Uchiha family, but also Konoha's history. If Boruto is the sequel, Madara would be the prequel.
Scar of Fullmetal Alchemist: Brotherhood technically qualifies as a villain, but the explanation behind his actions makes him a sympathetic one. Scar desires revenge for the literal genocide committed by the Amestrian military against his people, the Ishvalans. But with a mind mired in grief, he ends up killing Winry Rockbell's parents, two Amestrian doctors who were trying to help him recover from his injuries. Ultimately, he comes to the conclusion that hate begets hate, and it's his job to stop the cycle.
Not only would an anime about Scar give viewers the opportunity to get to know him better, it would also provide insight into the Ishvalan people before they were slaughtered en masse. Do other surviving Ishvalans feels the same animosity toward Amestris? And where did Scar get his training, and from whom?
If you enjoy being creeped out, a spin-off focused on Hisoka Morow of Hunter X Hunter would be right up your alley. To put it bluntly, Hisoka is a sociopathic murderer who loves bloodshed and other people's pain. He does whatever it takes to amuse himself, regardless of how it impacts others. Sometimes, that means helping people, but typically it means betraying them. Hisoka is unapologetically selfish, yet his relentlessly sadistic nature makes for a fascinating character study.
His terrifying nature might be too much for a series as long as Hunter X Hunter, but a 12-episode spin-off on his origin story could be amazing.
Some people would watch an Akame ga Kill! spin-off centered around Esdeath just to be able to look at her. Attractiveness goes a long way, but there is more to Esdeath than her looks. Although Esdeath is a social Darwinist who tortures enemies simply because she's stronger, she's also generous and fair to her subordinates to the point of inspiring deep and lasting loyalty in them.
This is a marked difference from the other villains in the series, who extend their villainy to their henchman. Esdeath's mind offers a fascinating place to explore, and a spin-off centering on it would be welcome.