I have no idea how to make an npc mod and I am hoping one of the wonderful modders out there will pick this up.
I love the Character Morgana Pendragon from the Merlin series. I think she could make a wonderful addition as an evil sorceress, probably only in BG2. To go along with the whole "magic repression" she had in the series, the cowled wizards could make a good story hook for her. She would probably be Nuetral Evil and will stay with you as long as she continues to gain power. She would have good dex, Int, and Cha. There is a portrait in the Super ultimate portrait pack for BGEE of her, however I think this should be made for the regular BG2, maybe BG2EE when it comes out.
If anyone would take this up, I would love you forever (in a non creepy way:P).