In Baldur's Gate II there are a noticeable lack of thieves for an evil aligned party. Two iconic thieves are rendered unplayable during the events of BG2 but remnants of their bodies appear as quest items that can exist long after they are gone.
MISCBU - Yoshimo's Heart
MISCBP - Bodhi's Black Heart
MISC6M - Demon Heart (3 possible)
MISC5U - Montaron's Body
MISC5F - Renfeld's Body
MISC5M - Inspector's Body
MISC7L - Valygar's Body
MISCBL - Aerie's Body
MISCBM - Anomen's Body
MISCBN - Jaheira's Body
MISCBO - Viconia's Body
Due to the contract Overhaul is under, Montaron and Yoshimo must remain largely untouched when it comes to reintroducing them back into the storyline. But what I'm suggesting is an NPC that has its own stats directly altered by the item the player chooses to use as the catalyst when creating the NPC. For example when using Montaron's Body you would create a NPC with 16 STR, 17 DEX, 15 CON, 12 INT, 13 WIS & 9 CHA who would be a Fighter/Thief. With Yoshimo's Heart you would create a NPC with 17 STR, 18 DEX, 16 CON, 13 INT, 10 WIS & 14 CHA who would be a Bounty Hunter.
The player would be offered to name the creation. So they could call it Montaron or Yoshimo to continue the storyline of these two NPCs if they choose to. However the NPC would be completely void of their past life's memories, so it would only be sentimental.
This would present an evil alternate pathway for the Removing Yoshimo's Geas quest, the Planar Sphere quest and the Removal of Bodhi's Curse quest. Whilst increasing the character choices for an evil aligned party. It would be limited to being done once. The ceremony could be conducted via a still living Xzar, a minion of Bodhi, something from the Underdark, by the player himself (provided they know Necromancy) or through details described in a book in Bodhi's lair (kinda like the anti version of the "The Vapiricus Omnibus").
The NPC's race would be something completely unique to them like Haer'Dalis' Tiefling race, maybe listed as "vampire". And their appearance would be dependant on the catalyst, so it would use a halfling sized body if created via Montaron, human sized body if created via Yoshimo, etc... It would have to be gender neutral if the catalyst used is female.
The NPC's storyline would revolve round the trauma of resurrection, finding themselves in Amn, their sworn oath to CHARNAME and generally getting along with the living. Possibly even atoning for their past life's sins, though to avoid the contract it would only be alluded to.
Anyways, all the above is just to illustrate the request, as there are many different ways this could be handled. The inspiration for this NPC idea was after seeing how the certain surprise in patch 2012 was introduced.