Exactly. I don't want the campaign changed in any way, nor do anything need to be rebalanced. Nor do I want some silly tached-on seperate multiplayer part just so they can claim multiplayer support (like Mass Effect 3 did). Just the single player experience, but granting a friend the option to create his own character and play it as one of the party members of the main hero.
Many people claim there are too many games with multiplayer support these days, but there have been forever since I saw a proper RPG game with proper Co-op support. This isn't about tacking on multiplayer to everything, but to bring back that good old fun co-op multiplayer for a game type that really fits with that kind of play. If you are going to control an entire party, why not let friends play some of the characters. Even if not everyone can play "a main hero", the game is still fun.
As for character interactions, these can still take place. Either by limiting the number of players so there will always be some independen't NPC's, or by still let the interactions happen even if another player controls the character. This is the way BG/BG2 did it. When you picked up a new character you met in the game world, you could delegate control of that character to any of the players, but any scripted interactions that character had with the main hero (or other characters in the party) still happened, independently of who actually controlled the character. In my book, this is perfectly fine. It ensures that nothiong is taken away ffrom the single-player experience, while simultaneously allows multiple people to play the came co-operatively.