You should release 1-2 new maps a month or every 2 months. It doesn't matter if they're mediocre or not, you just need a separate team to work on it. Every year, or half year, you should release a well drawn out map that extends the storyline. These inclusions would greatly increase the playability of the maps, instead of them getting stale after playing the same maps 50 times over.
 "SAS: Zombie Assault 4"){kind=icon}
 "Rebuild 2"){kind=icon}
 "TDP4 Team Battle"){kind=icon}