HTC is extremely picky about what player-built terrains make it into any of the three MA's. Indeed, I believe only a couple player-mapbuilders have managed to get one past the terrain gate-keepers at HTC. Basically, no custom objects are ever allowed in an MA map, and players are discouraged from trying unusual or innovative terrain layouts. While I understand the reasoning behind this, it means that most MA maps are simple variations on a theme. I'd like to see one day each week where, upon a late-war arena reset, a non-standard map is loaded in this MA for a set period of time (12 hours?). Perhaps there can even be player voting each week on which new terrain to try. HTC would need to review the terrains before posting them for comment/voting, so any obvious bugs or un-workable set-ups are pre-weeded-out.
There are so many interesting and beautiful terrains that we never get to try out under full arena loads. Things like custom airfields with grass strips and interesting defensive arrangements, bridges, hedgerows, flak traps, etc never find their way into the MA; I just think it's a shame.