There are a couple of classics that aren't yet included in the "official maps":
(1) Free Coasts
http://resource.openra.net/maps/5807/
(2) Texas Sea
http://resource.openra.net/maps/4720
Texas Sea is 5 player and there is only one of those in the official maps at the moment.
Hasselhoff is another map that used to get played a lot, (3 player), although the left island probably needs to be edited to give a little more room.
http://resource.openra.net/maps/4595
Any chance any of these become official?