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.
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Tomura Shigaraki - 'My Hero Academia'
The villains of My Hero Academia operate on motivations applicable to real-world struggles, such as loneliness and rejection. Excluded from society, many of them simply want to belong. Some suffer from mental illness, others blindly follow their own causes, and a particular few simply love violence. Tomura Shigaraki acts as a perfect distillation of all the other villains - he loves violence, nurses a deep grudge against heroes who failed to rescue him as a child, and he sure as hell fits in nowhere.
What's more, with his attachment to All Might's rival, All For One, he's a perfect foil for the series' protagonist, Izuku Midoriya. A spin-off series about all the villains in My Hero Academia, with Shigaraki at the helm, would offer a fascinating and compelling look at villainy born out of society's own prejudice.
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.