I think troops shouldn't cost upkeep on the turns they don't fight or participate in seige defense/attack. Possibly they shouldn't cost upkeep at all under any circumstances (other than food).
That way building up a decent pile of longbowmen or black plate infantry would be meaningful. What's holding troops back is the vast amount of money you have to pay them while standing/marching them around. If you can recruit them right into battle, even slingers are worth the price.
Also, I think that newly recruited troops should come into the game with slightly more experience over time. That way late in the game, a pile of black hunters is a little bit better than the black hunters would have been at the start of the game.
What if you wanted to play a game where there were a lot of battles with big armies on both sides? What would you change?
Oversway, the level 4 magic limit mod, in conjuction with SaberCherry's Recruitable Rebalance mod, has produced a lot of really big troop battles in our current game with it.