For a mystical sort, one who has a bit more sass than Doctor Druid, and isn't quite so busy with the Sorcerer Supreme-ing as Stephen Strange, I'd consider Daimon Hellstrom. His wife, Patsy, has been an Avenger, but he hasn't yet, AFAIK. Magik / Ilyanna Rasputina could also make for an interesting Avenger, IMO.
And yeah, if Songbird hadn't already been an Avenger, she'd be number one on my list.