A suggestion for ground combat:
So far there is little interaction between different components of a formation. If you separate out each in a different formation, or keep them all together does not matter. I would like to see more incentive to design combined arms formations.
My idea to accomplish this is to introduce a weight to targeting, depending on the skill of the defending formation commander. This weight is against hitting the priority target of the weapon, instead hitting a different unit in the same formation.
This would represent the commander employing the right units, tanks giving cover to infantry from machine guns, infantry covering the flanks of armor, and give a benefit to employing different types of units in the same formation.
A further expansion would be to also consider the mobility of a formation as a whole, instead of only individual units. Each unit would receive a modifier to mobility depending on the most immobile unit in a formation. This would encourage to keep assault formations and garrison formations separate, and also encourage producing mobile HQs for mobile formations.
It would further allow to have a purpose for a transport component. This component can be slotted on a vehicle, and allows it to increase the mobility of a certain amount of infantry in the same formation to the level of the vehicle. APCs and IFV are part of modern combat that is currently not getting represented in the combat system without any transportation requirements. This way you can add mechanized infantry to support your tank brigades, with a clear game play benefit.