Wheel of Time was doomed the moment it decided to depart from the source material and ignore the advice of Sanderson.

Here's the thing when it comes to adoptions. You CAN choose to ignore the source material or even change it. You can even ignore the spirit of the source material. But if you do that you risk losing the primary fan base. Which means that you really need to write the series as if its brand new and bring in your S tier writers, and acting. And wheel of Time didn't do that. They tried to pivot back in season 3 but the damage was done.

I still maintain it should be the witch king. We know that the 9 rings given to men empowered them and made them into Sorcerers and great leaders. And who is darker than the witch king?

The problem with being one of the blues is that yes they failed in their mission, but not till the 3rd age. They left middle earth in the second, and returned to valinor. Really that's what all of the wizards do, then they return in the 3rd.

Honestly what would be the most satisfying and likely acceptable to all fans is if the dark wizard became the witch king of angmar the leader of the 9 nazgul. This would make him a very very important character to try to derail.