A huge canyon somewhere! Like REALLY HUGE! (kinda the same idea that Iceman had with his old 171st base). But put it off in a corner somewhere.
Make sure there are a lot of starting positions, I always have a problem with lack of SPs on most user made maps.
Make sure our base has a lot of IFF 1 defenses (though you might want to make a version without them as well)... maybe make it into the side of a mountain with a tunnel start point (you would never have to worry about vulching that way!

) or just have some mountains around it for defense.
The runway and affiliated structures don't have to be anything special, copy the main one off the Hawaii map for all I care. Bunkers are a good thing to use if you can't figure out tunnels, pretty fun as well.
You'll want to have some ground targets as well, make sure they are a decent distance away from the base (about 100mi or so). Pop up a city with a bunch of buildings and such and a few starting positions here and there.
Also, I'm not sure how you do this but make sure that if you make any roads that you are able to actually land on them if you need to. It's kinda silly to try to land on a road only to die and find out it was classified as a field.
Another thought might be making a version where all ground is considered a runway, allowing ground vehicles and landing anywhere.
You odviously don't have to do all of that (though if you're up to it don't let me stop you

) I'm just throwing ideas out there.