I don't think there should be an "ultimate villain." I would prefer there to be various self-interested powerful people, and depending on what you do, some (or all) of them become your enemy.
I would also take it in this direction. Similar to Deus Ex 1 and 2, I'd rather have a whole bunch of complex organizations misrepresenting their true goals to use the player character, then have the player uncover bits and pieces of the truth. I'd rather it be Macbeth or Sin City. There are no "good guys". Just varying degrees of villains, and the player needs to choose their path and decide what sort of pain is left in their wake.
From everything I've read, Cyberpunk is not a game system about doing good deeds and becoming a "hero", it's a world about trying to survive in spite of everything trying to consume you.
Like I said, I don't disagree with you, but IMO, the story in Witcher is not very strong, since Eredin is not a complex villain.
I can sympathize with the body of players that disliked Eredin as a villain, but I think it has more to do with the execution rather than the actual character. I love the story behind the Wild Hunt and their whole purpose. I find it hard to fault them for what they've become, as their goal is, at its core, completely noble. Obsessive, misguided, self-defeating, hopeless, but noble. Therefore, I'm not really "angry" at Eredin at the end. He's pissed me off. He's left me with no choice. I certainly can't set aside the pain he's caused or the damage he's done, but I was feeling numb. Frustrated to the point of just methodically doing what needed to be done to end it. But Eredin isn't any more of an "enemy" than Emhyr is. In fact, they're basically the same person working to build different empires for the same reasons.
Eredin's execution (as a character) may have been a bit too "one note". Maybe a bit predictable. I would have approached the visual design of the Wild Hunt differently, myself.
I think things that looked like pure phantoms. Perhaps roughly human in shape. Maybe flashes of things that resembled faces or limbs at times, but otherwise ghostly forms that were impossible to focus on clearly. Weapons that were more implied than visible. If they wished to "reveal" themselves, they might briefly appear something like the distorted forms of the Nazgul from the Lord of the Rings films:
However, it's all just "technology". Literally magical costumes they wear. I would have loved for Eredin to confront Geralt and let the illusion completely dissolve. Pull off his "mask", and underneath is a tired, age-lined elf. Without the vocal distortion, he sounds much like an older man, growing perhaps a little infirm. He's regretful of the hardships he's caused; haunted and tortured by the choices he's made. But, still obsessed with following his vision through, regardless of the cost to himself or anyone else.
However, it's all just "technology". Literally magical costumes they wear. I would have loved for Eredin to confront Geralt and let the illusion completely dissolve. Pull off his "mask", and underneath is a tired, age-lined elf. Without the vocal distortion, he sounds much like an older man, growing perhaps a little infirm. He's regretful of the hardships he's caused; haunted and tortured by the choices he's made. But, still obsessed with following his vision through, regardless of the cost to himself or anyone else.