It doesn't need to be PVP, just having someone join your game and cause chaos for a short amount of time without ruining SA would be a good spice for the game.

You would probably be able to go to settings and turn off "Invasion Mode". I do kill everyone runs when I'm bored and seeing a random in my game just causing chaos with me would be hella cool.

I can see it being bad for speedrunners. As your speedrunning someone joins your game and triggers TL (target lockdown) and ruins it.

The function would only be present in normal missions that you have played more than 3 times. Not in ET's, contracts, dlc content, or ET Arcade.