AH2 added the small hills, trees and such that AH1 didn't have. Since then reflections, specular mapping, shadows and bump mapping have been added. Per HiTech the current engine is fully capable of any level of graphics they want to do.
What I would like to see is a much higher polygon count terrain map which would dispense with the need for the small, repetitive hillocks that are pasted onto the current low resolution terrain maps in order to give the tanks something other than large, flat, treed planes to fight on. The problem with that is that it would immediately remove all current maps unless the old maps could be loaded in the new version. The old maps would still look just as they do now though.