On an unrelated note, I really hope we get Custom Game Browsing and Dedicated Servers for Halo 4.
I reckon Halo 3 would be much more active now if it had them from the beginning as they allow communities to build as you'll get people becoming server 'regulars', thus forming long term friendships within the game. In general they'd provide a much more user friendly experience for newcomers as well, providing betrayers and trash talkers can be kicked by hosts. I think they'd also be magnificent for the competitive and creative communities, Custom Game Browsing would allow good forge maps to spread like wildfire. For the competitive community, dedicated servers would be much more useful for hosting scrims, as well as handling general clan recruitment.
Overall, there's no other two features that I think could massively benefit the Halo multiplayer experience as much as these, I've expanded much further on this over at Halo Waypoint so if anyone would like to take a look then feel free!