I'd like to see some town variation upgrades to teh game before we add a lot of various new ground AA or AT guns. I'd love to see both come in at the same time.
IE: Have some small country-side towns/bases with some medium guns and a lot more lighter guns, have medium sized citys/bases have a good variety for its size (one or two large/heavys, a handful or two of mediums and peppered with light ones, the more important/larger the base/town the more it has) and have London and Berlin sized towns be bristling with large, medium and light guns.
This could also help in avoiding having a critical base taken by one or two lone pilots sneaking in around the back. They can try, but they'll have to be either determined, brave, stupid, lucky, or all the above to have a chance in getting it done.