What I'd really like to see is for it to model the historical realities a lot more.
1. Instead of regional troops, just make generic recruits available. If you try recruiting in an area that's hostile to you, you don't get your quota filled very quickly. The larger the number, the higher the amount of money it should take, not just linear but with a bit of a curve; larger numbers means more logistical problems, more trainers must be provided to create cadres, etc.
2. How they get developed is up to the player; how long it takes depends on what the player wants to do with them. Basic infantry barely good enough to stop bullets with might take a few game days, really elite cavalry might take a month.
In terms of how the feature would work, if, say, you want to teach 100 men how to be basic infantry, you'd pay a local recruiter, or pay a small amount of graft to the local villages to find recruits. Instead of hiring them straight in, like Warband, I think it would be much cooler if you went to the Village Elder, paid them 100 dinars, said how many troops you were looking for, then after some time you'd have the troops show up for training. The more villages you contacted, the faster the quota would be met; with recruiters, you could give them a bonus per head, and the higher you set the bonus, the faster the quota would be met.
Then they're in training; instead of putting them into your band, put them into the game as variables, shown in a menu. You have to wait for them to be trained before they'll show up in your band. Depending on what you want to train them to be, they may take days or even game weeks to show up; realistically, it should take months to years, but it's a game; I wouldn't make it that literal, heh. Factor in Trainer, in terms of time; your character isn't there, training them, but Trainer would represent your ability to give clear instructions to your employees and pick the best trainers.
When they're trained, trigger a Dialogue where the player can choose to take them into his / her band, or have them put into the garrison of one or more locations. That's probably the hardest part of writing it, from a gamecode perspective.
This would be a great new feature. It would mean that you could pay a recruiter to find the troops, but it would cost more, or go from village to village, which would be cheaper but take up time. Then having them auto-train outside your warband would save a lot of messing around, and let people play the game a lot more strategically; perhaps they'd fight a series of holding actions, knowing that a unit of heavy pike-and-shot are coming in a couple of weeks, or they could start a major offensive, knowing that their investment of 50,000 would result in hundreds of mediocre-but-plentiful garrison troops in a game week, allowing them to guard the castles they're planning to take.
In short, make the Warband system a lot less annoying and micro-heavy, and get rid of so much pointless roaming around. This was a period when raising armies was perfectly practical, because firearms training and learning the basics of being an infantryman are a lot faster and much easier than creating a knight, and inventions like the printing press and the rise of literacy made it possible for messengers to contact distant villages and round up men willing to follow the drum.
As for "a serf should always be a serf", handle it by allowing for recruitment of upper-class troops, particularly cavalry, from the Fortresses, giving them a similar role. It's perfectly reasonable that certain types of troops can only be recruited from the upper classes of this period, but the process should be about the same, just with a different cost structure. I wouldn't go very far, though, with missile troops; if anything, the elites should be commoners, not the upper classes, when talking about this period.
Remember, anybody can use a gun, and the commoners, not having silly notions about the glory of charges and the honor of cold steel... probably made the best line infantry. This was a period where the mounted knight was almost dead, due to technological changes; commoners were all of a sudden a major factor on the battlefields, and the game should reflect that.