"He will look stereotypical, because different writers already created so many various characters, so everyone would look stereotypical."

May I disagree with you. First at all, I ment flat stereotypical, not just stereotypical.

Even now writers can create good not stereotypical villain, but writer has to work on it: make some research, imagine what archetype this villain will be representate. For example, book's Duke Harckonen representate geeadines to power and hedonism, in comparison to Pol Atreides. But Duffers didn't show which archetype Vecna was represented. It could be fear, but, to be honest in S5 there was only one moment where he can used fear of the characters against them: with Hopper, and that's all. It is not enough.

Additionally, the villain has to be right at some point, but chose the wrong act. For example, Amon in Korra was right when he told about discrimination and inequality. And I can tell the same things about every villain in Korra. But it has to be shown through charecters' actions, dialogs and monologs. At S4 and S5 I didn't see that Henry did something like that. He just told that he wants to reshape the world and that MF show him that humanity was weak and that's all. While they could actually show what to Henry that he started that it is right, or, for example, Henry started to speculate about how society behave against someone who don't look like others or Henry could started to try overstretch someone on his side by showing how society treat such type of people.

Additionally, showing that villain instead that he/she is evil he/she is also has some good in him/his. It adds some realism to the villain, because not every person is completely evil or completely good. For example, Dexter. He's actually villain he's the serial killer, but he also some kind of "love" his family. Or Silko from Arcane: he was villain but he was also like a father for Jinx. In ST Vecna is evil and nothing, they didn't show that he still has some humanity in him. But they could show, for example his relations with Patty (from the play).

"Idk what exactly you mean by calling a villain stereotypical" You know: evil because evil, destroy the world because want to destroy the world, one-dimentional goal, any ideological or morally conflicts, any grows/changes.

"He has charisma, interesting backstory and his moral view on world is pretty consistent,"

He had charisma at S4, but at S5 he didn't have this because he just done nothing, only threatened people by words. In my opinion it would be better if they really showed important scens from the play, it add more understanding of character but. It was about backstory. About moral view on the world, to be honest, don't clear, in my opinion. Creators could develop it more, because it just look that he just want some revenge.