We did have funner and more expansive maps in H 2 and H3. There are three story maps in Reach called Sword Base and Boardwalk, but I understand you desire for an urban style map with multi level structures. The game typically needs to stay fast paced and have to take into account that given a place to camp an area from a room or stairwell, certain players will sit there the whole game. Halo, in the beginning always lent itself towards movement and holding strategic areas. Reach has created the "camping" technique brought on by two or three reasons. One, Invisibility, armor lock and other AA's promoted it. Two, some of the Reach maps design and weapon spawns and static spawn time ensured it. Third, new, inexperienced and achievement chasers used it get easy kills and sprees. This is not global, but has become a real phenomena in Reach.
A map with more players would only be more confusing and laggy, in my opinion. I have not interest in Big Team so my opinion may be mute on the number of players.