Well, I hope you realize what you're getting into. If you start down this path, you'll wind up with a game that's no longer mount and blade. It will eventually turn into something like Victoria (2?) but with a 1300s tech level.
Also realize that this sort of thing requires HEAVY testing to work out. It will take a very long time, and a very large amount of work to fine tune, let alone implement. Every single thing you add in will destabilize everything, EVERYTHING that's already there. It's like trying to build an entire world on a platter that's balanced on the tip of a needle.... but worse, because the platter glitches out on occasion, or doesn't recognize the needle is there, and so forth.
My suggestion for this though, if you do choose to take this route, is that you create a population system from scratch. A very simple population system that's based on a needs(to survive), wants, and hates system. This would decide how the population wants to move around, with distance being an balancing modifier for this. (Noted, I haven't looked at your pop code yet, so I don't know how your current system works... Well, I think I looked at the old SOD system a few years ago... A month or two before the strategic map was implemented. Doubt it's the same, not that I remember any of it.
)
Needs would be basic trade goods produced by towns and villages. Basic foods (primarily grain and bread) and cloth, wool or linen, would be needs. This could also give you something to do with cattle to keep it from reaching insane herd sizes. Possibly pottery and salt, whatever. Healthcare as well.
Wants would be things like chicken, pork, velvet, spice, luxuries and tools and such. Furthermore, all political aspects would fall into wants. This could include relations with a town and it's lord as much as freedom/serfdom scales. (those scales would also influence a people's ability to leave their towns. High freedom = high immigration, but high serfdom = low emigration since it's pretty damn much illegal. ) If you were to implement different casts for populations, then aristocracy would attract more noble families as well. Dominant faith could also play a role.
Hates would pretty much be more abstract things. Low health of a town, being raided, constantly changing ownership of a town, too many damned Swadians living there, ethnic hatreds, being a minority.
There would also need to be a breakdown of each culture as well, as well as an extra one such as Cosmopolitan. Each one would have slightly different needs, wants, hates and so forth.
Also, to account for population growth, there would need to be a population pool of people who immigrate from Calradia and other places, which slowly trickle in. This would help to offset the masses of people who are going to die from your new system of population.
If you need help, I might be able to spend some time balancing it. I'm rusty as hell and haven't done any real coding in years, and hopefully will be moving soon so I might be away for a time, but I might be able to help.
I might also wind up being intoxicated a lot as well. It's the only thing that makes me remember how to do this stuff, since I was never sober when I did this stuff in the past.