An urban map would be spectacular to have at some point. Big City was one of my favorites back playing MW4. The biggest problem I can see with designing a city map would be deciding what to do with the buildings...in MW4, there was no such thing as a battle armor (MekTek packs aside), so all the buildings were just big boxes. With the addition of infantry-scale combat in MWLL, it would be nice to have at least some, if not all, of the buildings interactive on that scale, ie, entrances, exits, and various floors with spots from which to ambush mechs. Not difficult to actually put together, but that much additional architecture would probably be a massive resource sink.
And you can't go ruining the view distance! What about street-sniping?! Also, would mechs JJing on top of buildings cause problems?