Re: If a new expansion could be created...
Very good idea!
Personally I think a staggered hit point and armour bonus for dark-imperial age would work well. So as is now imperial age buildings would be the same. Then maybe reduce it 15% dark 10% feud 5% castle. Would need to be tested.
Also could reintroduce stone for masonry and architecture and hoardings, as it was in AOK.
Very good idea!