How about a naked balance on our current classes?
Aside... You "100% agree" with a comment about adding Blood Magic in the form of a Blood Mage.
Cultists are commonly what we fight. They aren't really a class, like the Necromancer. They are more of a description. You will have to elaborate more. If your intention was not to make them a follower of some manner of Blood Magic cult, then what are they?
If you are going to suggest a "Hexer", you will have to define that, too. I have seen it described as something that uses curses based on blood magic. It is also a translation for "Witcher", as in Geralt, which takes it off in the different direction of Signs. If the idea is to do Witcher in Elder Scrolls, I would suggest a "no" to that. By the time the lore was made to align, they would just be spell sword that is part of the Vigilants of Stendarr cult or Dawnguard.
As for doing something that has not been done in the series, the basic answer to that frequently seems to be pulling some special thing out of another game. Elder Scrolls sort of takes the generic middle ground in this respect.