Also, population could provide a handy "force limit". By recruiting from the population, you decrease the number of workers, so your income decreases. It will be up to the player to find a balance - the larger your army, the more maintenance you will pay AND you will also get less income due to a smaller number of workers.
This would be very interesting if combined with the ideas in the other thread about if the armies consist of standing professionals or raised levies. If you rely on levies your regular income would be greater, but long wars will be very costly. Professional soldiers would equal a more even budget instead.