Lanchester's square law is very important, so what I'd like to see is battle going on in stages (it was like that in warband kinda), with no reinforcements. Like if it's 1000 vs 100, and the battle size is 500, then the initial battle would have to be like 45~ vs 450~, to keep the ratio of forces. Once this ends, next stage starts with fresh troops, and updated ratio. Troops that were already used and are wounded, could be kept in memory and sent to the back of the queue. Then let's say the 100 guys are uber elite, so they inflict 200 losses while losing 10, the next fight would be based off ratio of 800 vs 90, so 56~ vs 444~ giving a better force ratio.
Making tactics a factor of turning numbers in your favor would be nice, also the ability to choose which troops to send. This could make tactics skill pretty interesting. Medicine skill might heal or have a perk to heal those wounded troops that are in the back. All kinds of things could be done to make this interesting.
Removing reinforcements and replaced troops with fresh ones would make more sense too, as IRL you would want to send exhausted troops back anyway, and historical battles often happened in stages, with only part of the armies engaging, and this could last for days.
Reinforcements could be kept for sieges though, defenders advantage and all that.