Greebo has given good advice and I can add only a little to his recommendations. I'd say think about the fights but also the types of missions a new squad to AH might try to run. Old squads will know exactly what they need for success in any aspect of the game, but new ones are something else. When I think of an MA design, I want some things to look easy, and inviting. Okay, it's a trap for the unwarry but guys love to run NOE if they think they can capture a base. It's exciting, and even if they don't succeed, they can have fun while providing a fight for the old pros. You can always count on some fool breaking dar and annouuncing an NOE attact. Even if that doesn't happen, if a country can't kill a Goon then they deserver to loose the base.
I can't swear to this, but I remember htc posted 6k as the maximum alt for any airbase. To be safe, I'd stay with 5k as max and want one or two bomber bases at that alt for each side depending on the terrain size. Make sure you have an equal number of 'high alt' bases for each side or you end up with sfma's two. One is at 6.0k and one at 6.6+k, but none for the south country are above 1.5k. btw, mindnao's A2 is at 5k and A54 is at 5.5k. The Baltic terrain had one at ~6+k but I haven't looked to see which one and how high it actually is. It exceeded the current hight limit iirc.
My personal priorities are that a terrain is fun and attractive, but that it would be hard to reset in under a day (lots of bases but there are rules for that). Then I think about the types of missions and fights I want to see, CV battles, GV battles for territory, in addition to the usual low alt furballs, etc.
Everybody will have a different concept and that's what will keep the game fresh.