My hope is we don't have villains associated with others. Given that, I'd rather not have the Entropic Man or Modred (the former for Swamp Thing, the latter because of his connection to Morgen Le Fey). Same goes for Magus.
Kaluu is one of my first choices. It's an extremely obscure choice, but it fits the description. The Imperator of Empirikul (aside from needing a name) seems to run into a problem where I suspect first drafts of the script predate his existence in comics.
I've heard suggestions of Kaecilius, which would require significant changes for who he is since he was basically a follower in the comics.
Really, there's a lot we don't know. I'm sure whoever it is, unless it's Nightmare, isn't going to be well-known to anyone, but I'm sure that'll give them flexibility to just do whatever they want to fit the movie.