Well for immersion, if this type of system were to be implemented, CA could take a few notes from Hearts of Iron. Where you could have Lieutenants/Captains/Generals/Admirals etc. start with a fixed "tactical value" at the beginning of their careers, however depending on experience and overall set abilities (like the fixed personality traits in EB) would determine how successful of a commander they are and/or could become.

The Hearts of Iron type system could also add a level of complexity, where you might have a Highly Skilled and decorated Captain whose good at commanding two or three units, but would suffer severe command bonuses when leading a bigger army. However, if the situation demanded a field promotion so that this skilled Captain could lead a bigger army, the disadvantage is that it would take time for the Captain turned Maj. General to learn the necessary skills required to command such an army, although over time, and through experience he will probably become just as effective as he was before the promotion, You might suffer a bit in the beginning, but the advantages would probably pay off, or you might have just wasted your efforts on a perfectly good Captain. But thats just my two cents.