So who's left from the earlier seasons who hasn't been brought back yet? I can remember...

Shannon (probably isn't coming back)
Juliet (probably Jack's kid's mother)
Eko (doubt if we see him but it would be cool if we did)
Walt (Walt! Waaaaaaalt!)
Nikki & Paolo (lol)
Vincent (if they kill off Vincent I suspect Miriel will erupt in a volcano of fury)