I imagine this is at the top of a lot of peoples' wishlists, mine too, so I apologise if it's already been brought up and discussed to death.
I think it'd work best with a lot of elastic progression elements:
The player-kingdom Troop Tree is derived from that of their kingdom's parent culture. However, the trops are of a generally worse quality skills-wise, and their equipment is knocked down a tier, with negative modifiers.
The player can hire trainers to improve troops' skills (companions/governors/vassals could also serve the role). If the player terminates the trainer's employment, the troops' skills regress. Skills/focus/attributes could be automatically assigned based on the troop's role (per their gear), modified by trainer quantity/quality, parent culture, and maybe a couple levers for the player.
A bonus to the effective training level of kingdom troops in one's party could be added to heros' Leadership skill tree.
The player can invest gold to upgrade the troops' equipment sets, allowing them to freely choose weapons and armour for each troop, within tiers respective of their rank. More expensive gear makes them more expensive to recruit/level up and increases their wage.
New equipment is rolled out gradually over a couple in-game weeks, during which some of the troops of a given type will still wear/wield the old equipment.
Towns would have equipment workshops, which determine the average equipment quality modifiers of the kingdom's troops. Again, heroes could play a role here with high-smithing characters adding a bonus. The player can also invest monetarily here, to keep those imports flowing.
Demand for the comoddities needed to produce each piece of equipment would be reflected by the materials needed in their production - affecting price of troop upgrades and wages and the city/kingdom's economy.
In summary, the quality of troops and their equipment is just as much about the kingdom's economy/manufacturing to support them, not just which equipment you choose for them. The player may find themselves having to downgrade their troops in some way to keep costs down or to stabilise their economy.
That's all very intricate but just one example of how it could work and how much it could add; Better player expression, more moneysinks for the player - The Warband mods that feature it are an order of magnitude more enjoyable for it.