Sometimes, I play alone in a SP scenario or campagn, and a friend watch me with steam streaming, or inverted. He can talk to me via steam chat but it’s not practical for the player.
It could be cool to be able to join as a spectator, if the player allows it (only friends maybe ?) in a multiplayer map (even after the game is launched), or even in a single player map, and then allowing in-game chat !
I think it could be a good addition to the game…