I like that kind of advancement, regardless of realism. However, the beauty of AW, at least for me, is in its simplicity. So I don't think there would have to be deep trees necessarily. But why can't I build an airport instead of starting with one or finding one in a map? Certainly it would seem realistic that I could build my own. So maybe a shallow building tree, and a couple of upgrades...some that can be manufactured at a building you build, maybe some can be found.
Wouldn't it be cool if your units could level up? Maybe level 3 is the max level, I'm not asking for a huge character development system. Just a slight advantage that would really reward good unit utilization. Maybe they could find a bit better of a machine gun if they explore an out of the way building (forcing several turns or use of an APC).
I do really like that capturing resources is a big part of the game, I just want a bit more. I also really like how offensive play is rewarded in AW...but perhaps there could be a new unit or two that excel at defensive maneuvering? They could suck at attacking, but really do well when attacked?