I had an additional idea for ground combat. Preferential targeting during combat leads to favoring monolithic unit compositions and army compositions to minimize damage, but I was thinking what about preferential targeting against optimal weapon matches during the formation targeting phase, but unweighted targeting during the actual combat phase?
This would represent commanders picking good matches for engaging formations, and it would lead to favor mixing up units, as otherwise your tanks will face mostly heavy guns, while your infantry runs into heavy machineguns. For mixed formations, there is no obvious optimal match.
The bonus could scale with commander skill, making well led armies taking better engagements.