The Witch could be part of her own faction, or part of a peasant faction(yes, more peasants please). Basically I'll wait and see before deciding. I have had fun designing male and female spell users for the Undead faction so another witch would be ok.
Succubi? Although I've wished for them (for years on end) and other "demons" I seriously doubt LEGO will make that as a toy for kids. I remember my brother making the comment that LEGO should team up with D&D to make monsters and such but of course this never happened as some parents would try to burn LEGO "at the stake" as it will.
Elves should be a stand-alone faction so that there will be more sets in the range. They will come in their time so I'll spend more effort to enjoy the current sets as they will be gone too soon.
-Tower