Guinan, I think there would of been a lot you could of done with her character, she was interesting to start with and there was a lot missing from her back story. Plenty of room to explore.
If Worf hadn't gone to DS9 he could have been assigned to another starship, perhap as first officer and in charge of away teams, he would of possessed he's own style and still you would have seen hints of his previous shipmates in his decisions Given his rank he might have been place in command of a small starship of his own (Defiant size). Third possibility, Worf could be Court martial out of star fleet, become a wondering ronin within the federation.
Wesley, not everyone's favorite, his situation the last time we saw him on the show would have lent itself to a interesting show.