I prefer the idea of going back and forth until the final battle. Though it is also fun to "follow" the guy around until the final battle. I prefer to know who you are battling....however battling 2 foes is good too. It takes your mind off of one and then bam right back at it.
I don't like the idea of a member of your party going bad. Unless the person goes early...or he is just not likable.
I think in the case of Sephiroth he was just so damn cool looking and bad ass. But then...it got out of hand and you hated his guts.
What would I want to see...
Male: Should be hated by almost everyone. I prefer the bad guy to be mysterious. Not much is known about him other then they are bad. A loner with no friends other then his belief. Though quiet and powerful friends are good. Depends on how evil. A loner has the ability to be much more cruel because they show no affection towards anybody. I don't like bad guys who talk a lot of shit. Though maybe that's good?
Female: Good looking
I prefer a sorceress...though I have no problem with one that wields a sword. A female companion that is more caring yet is still loyal to the death. She can be famous or not.
I guess those aren't that original.... but it's just what I like.