As HT said you can't build your own airfields in an MA terrain, however there is a new super-large airfield with a built-in town in AH3 that you can add. I haven't counted how many hangars its got though.
If you are designing an MA terrain there are various HTC guidelines regarding that in the sticky post in the TE forum. I have a few of my own suggestions about it.
The terrain should be fair to all three sides, players shouldn't be saying "Oh no, we got the north side again". I realise this does not always apply to existing MA terrains but if you design an asymmetrical layout you should be prepared to justify it to HTC. Personally I think the layout should also be fair to each type of player. So GVers should have a reasonable selection of spawn routes to choose from and there should be a good chance of CV action somewhere.
An MA field can have up to 255 fields and be either 256 or 512 miles square. Something to bear in mind is that to win the war a country has to capture 20% of each opponent's fields. So for a 120 field map that means 8 fields from each side (0.2 x 120/3). To add one more field per side to the win requirement you need to add 15 fields in total, so 120, 135, 150 and so on up to 255 which requires 17 fields per side. It also means that the fields towards the rear of your map are less likely to be fought over so you should put the most thought into your forward fields' layout.
The biggest difference between the various MA maps is in how many front line fields are available to be fought over at any one time. The more choices of nearby airfields or available SPs a player has, the more spread out the action becomes, especially at off-peak times. Personally I don't like maps that have more than four choices of field per front line per side.
Airfields should be between 18 and 25 miles apart and not higher than 5K, bombers struggle to get airborne with a full load if fields are higher than that. No other type of field should be closer than three miles from another. There should be a reasonable capture path for each field; either a nearby airfield, a spawn point into it, or access for a task group.
I'd suggest putting each HQ and city reasonably far back from the front line. The three fields nearest the HQ are always uncapturable and one of these is the Me 163 field.
Once you have your layout worked out send a drawing and a description of it to HTC and maybe post it in the TE forum. You should point out anything on it that is unusual, i.e. that you have not seen done on an existing MA terrain. Generally if it hasn't been done there's a reason why. You should do this before you start serious work on it in the TE as having to redo work later is very off-putting.
