I read or heard somewhere that if the player is good enough in a skill people may ask him for training. So here's my idea, the player can invest money in the area, and he can find new recruits and train them if he wants.
this way he can earn money not only by fighting himself, but have team fights with more people in combat. If you have trained your men well your team wins and can go up in ranks. I think this could be a really satisfying element in the game, do something different you know.