Yes.
The ability to kill every NPC we come across. Bring back Biff the Understudy and his clones in case it's a plot critical NPC.
And if you're worried that it might be unimmersive/breaking the 4th wall, I have two arguments against that:
1) it's not more unimmersive then untouchable/invincible NPC's
2) players that would kill any PNC (especially plot-critical ones) are well aware of what they are doing, and are willing to the pay the price of 4th wall breaking
You're asking for the impossible, preaching to the choir. In Baldur's Gate 2, killing a plot-critical NPC will result in an NPC attacking you that is completely invincible, so... no.
