Enemy surrender in action games. I remember being amazed seeing enemies give up in games like Soldier of Fortune and Rainbow Six, and of course SWAT 3 and 4. I thought it'd become more of a thing in the future as AI got better and enemy behaviour got deeper, but it seems like the opposite's happened, and nobody's even tried to play around with this type of mechanic in the last twenty years or so, barring stuff like Ready or Not which is just a regurgitation of the exact way SWAT 4 already did it. It's so dumb to be playing modern games that aim for "realism"/verisimilitude, but then you've still got some dumbass with a knife trying to kill you after all their friends have just been one-shotted and you're wearing the best armour in the game.
RPG developers in particular have really missed out here because it'd presumably be relatively easy to implement surrender and capture mechanics to let people roleplay as bounty hunters and turn captive enemies over to guards, but nobody's picked up on the idea except for a couple of New Vegas modders. Bethesda have missed a trick as usual in that Skyrim and Starfield actually do have enemy surrender, but all it means is that enemies will stand back up after ten seconds and start fighting again until you're forced to deal a fatal blow.