I didn't mind January Jones, but I wouldn't really care if she didn't come back. Same goes for Angel. I think the only ones that I would miss if they didn't come back are the big characters who are pretty much guaranteed to come back.
If they do decide to go into the '70s, they could do Dazzler. I've never actually encountered the character in any of the stuff I've seen and read, but I think the idea of a '70s rock star mutant could be alot of fun, if done well.