I'm with UnseenWombat, I'd prefer no more guests and just more SC characters (be it new or past). But like another user said, I know that they make the game more profitable so well hm... I'd be for anyone, so long as they fit the time century of the game or relatively close to it. Star Wars was just a joke to me but I'll admit, the Apprentice was fun to play as, they should have kept and re-did some of the moves for a new SC warrior in my opinion. But yeah, I wouldn't like to see more like them in the future...
For more recent game choices, I'd choose...
-Ryu Hayabusa (Ninja Gaiden series): He's probably one of the few modern warriors I'd like to see as a guest in SC. He's a pretty cool ninja in my opinion and I think he would have an awesome moveset. Plus, he battles supernatural enemies so I can see him fitting in quite nicely. Maybe a time travelling demon opens a portal during a fight against Ryu and sends him back to the 16th-17th century in the Soul calibur universe. :)
-Bayonetta: At first I didn't want her in but now I'm slightly partial to seeing her. As long as they kept her pistols to Critical Edges/grabs and do a fair amount of damage, I don't mind. And she can use her hair as part of her sword combos and Critical Edge/Brave Edges. :)
-Dante: Like Ryu Hayabusa, he is one other modern guest I wouldn't mind seeing as a guest character. Yeah he won't fit the timeline but I do imagine he'd have quite a cool moveset. Like with Bayonetta, they should definitely limit his guns to unblockale attacks (like Cervantes), Brave Edge Combos and Critical Edge.
-Yoshitsune Minamoto (Genji Series): He'd make a cool guest I think. Though the game isn't all that popular...
-Nariko (Heavenly Sword): She'd make a great female guest!
-Alucard Belmont (Castlevania)
And yeah, that's pretty much it for now. ^^;