I like the idea of mercenaries. Perhaps when you recruit mercenary units you could be given the option to customize their equipment in some fashion? That would be kind of cool for the players.
You could tie that in with quest rewards and crafting systems that would allow unit customization of hire mercs without compromising the integrity of the standard army choices.
Recruit basic dwarf mercenary. You could either pay a higher cost and he comes with his own standard equipment or you can to borrow a phrase "have him take the emperor's coin" means you pay him a basic fee and then provide custom stuff gotten from quests for crafting such as:
Give each weapon a specific stat just like an RPG and let the players create their own special additions for their armies. Make em limited to a certain number of custom mercs per army to prevent overpowering force.