It would be cool to be able to start multiplayer games without another players and have an option to join at some point to some already running game. That will make finding the game to play so much easier.
Or at least to be able to select your side while loading multiplayer game in single like you can in multiplayer.
That would be sort of fun to have an option to spectate in single game too. I can assume it is not much of effort just make it like the player was defeated from the beginning. It is good for mod testing. It can be enabled by Modding Tools option.