I'd like to see Lancelot because deserves a better ending than what he got and Nimueh she was clearly the most powerful of the High Priestesses she could summon and create monsters raise the dead and throw fire balls I also can't help but think she was the most sympathetic villain. I mean she did what Uther asked of her and even stated she never would have done it if she had known.
Besides I want to see how the Merlin of today would interact with her. I mean would he completely dismiss her views or would he conceded that is right to some degree. That and I feel like they are teasing us I mean how many times to do we get a portal to the underworld open and even spirits that escape? She was also smart and figured out the truth in no time.