Definitely think this would be a handy feature, another example is when someone leaves- I think it could be nice if they 'tag team' in. Like if you're in the MSQ roulette and some people leave because it's the wrong one, NPCs could take over and then players can still replace those npcs (with perhaps restrictions on like "if currently in a boss fight" lol). Also these NPCs can do a unique concept of instantly replacing someone who was dropped, like due to lag, and then let the player return with their slot saved with the team being able to choose to have the game search for another player.

Huge opportunity to abuse such a tag team system so it'd need to not work for the hardest content, but I see no problem with it in story level content, even if players 'abused it' .. whatever.. it's story content.. if that floats your boat lol.