With the kind of graphical update we're looking at with Reach, I think its kinda unfair to expect more players/bigger maps in MP. You can't have it all.
Maybe Halo 720 will support 24 - 32, but asking a 360 game to do better gfx than H3 (which was pretty good) *and* bigger maps/more players is a bit too much. Bungie knows Halo. Let em do it right, and don't whine about the MP player count.