I for one would love to see customizable armies and icons. However, I also have a few ideas I believe are unique.
-perhaps most importantly a remodeled menu, I see lots of new players simply fail to navigate it and quit a great game before their adventure even starts.
This one is kind of a stretch and all hypothetical:
-perhaps a currency system based on performance where you can hire real historical generals that will provide morale boosts to any troops nearby. For example, a really good one might rally the surviving troops to attack, where they would normally flee.
-with customizable armies, it would be great to have experience as a factor. For example, there could be three tiers of troops: green/Unexperienced, hardened/regular and grizzled veterans. Their cost to the army build could be affected by the experience of the troop selected.
-being able to name units and reuse the same ones who survive battles (obviously refreshed for the next one). Maybe each individual unit could have a name (optionally) to see them more as real, historical people and less as just temporary colors on a screen. It would be much more heartbreaking to see my favorite archer unit get reduced to nothing, or to see my grizzled cavalry smash enemy lines. It would also add somewhat of a historical value to the whole battle.
Thank you for a great game regardless