Ideal: YOLO

Player characters might not live once. They might live three or four times, being resurrected through powerful magics when bad luck or poor tactics should normally have ended their tale. Very little can permanently end the adventuring career of a mid to high level adventurer. After a certain point, even being reduced to being an extra finely ground powder isn’t an impediment.

So what can permanently kill a PC?

One simple thing: that player having a cool character concept for a replacement PC.

That makes the immortals mortal. Nothing can save an adventurer when there’s a build begging to be tried, a character desperate to be played, or a concept the player can’t get out of their head.