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
I recall a presentation (or Q&A) with Swen about something they call "n+1" or something like that, which basically means there are several ways to solve a quest (n) but there will always be a fallback option if you decide to kill everything (+1). Think of it as finding scrolls with information, or maps that lead you to the next location. Although BG3 might have a different narrative structure I think this is a great philosophy to have.
