How killer awesome would it be if you could observe any game in progress? Find a game, see who's playing, and load it up as you would a replay... except it's live...
I dont think it would have to be anything fancy... same options as replay has (except the fast forward... duhhhhh), and chat disabled for spectators...
Lately, especially when most of the people i usually play with are in game, it would be hella cool to watch their game instead of just idling in lobby...