Horses and wargs as a resource
The way that mounts work as a resource and upgrade seems lacking to me.
As a resource - other resources remain important for the entire game. Crystal and metal are critical components in unit upgrades and unit production. Mounts on the other hand are abundant. Producing a mounted unit takes only a few mounts and will never need more for an upgrade. I always end up with hundreds and hundreds of wargs and horses with nowhere to use them. Another side effect is that there is no reason to create foot troops once you have mounts available - there are only advantages with no disadvantages.
An option to burn up this resource would be something like a movement bonus option for armies. Give players and AI an option to use up some mounts from the resource pool to give a movement boost to an army. This would make all those excess horses and wargs have some strategic value.