What I've been playing around with in my game is the idea of treating certain game elements like magic items. That is, if you build certain elements related to domain play, you get the ability to do something you couldn't do before, which might have a mechanical benefit attached to it. With the idea that players will covet those the way they covet magic items.
For example, if you clear an area, build a castle and start attracting peasants, you become a Marcher Lord, which affords you a certain status. This gives you a bonus to social skill checks reaction rolls in situations where social standing is relevant, and to loyalty rolls. If you get more land or increase your standing in some other way, the bonus increases.
Building a castle grants bonuses to morale checks of troops stationed in the area. Building a spy network opens up certain types of rumors, and also helps with certain types of knowledge checks can provide information like a sage with related fields of study. Building networks of domestic workers or street urchins open up other types of rumors and help with different kinds of knowledge checks provides information like a sage with different fields of study. Building a counterintelligence network prevents other spies from getting similar information about you.
Other lords can plot to reduce you standing, and thereby reduce your skill reaction roll bonus, by engaging in similar shenanigans. They might pretend to be bandits and raid your lands, stealing resources and causing famine, which lowers your standing and causes you to lose peasants. Or they might engineer or manufacture a scandal and turn the king against you, or sabotage your grain storage, or incite your peasants to revolt, or blackmail your castellan into stealing from you. The currency here is social standing, but unlike honour point systems the players have a reason to care about it because it gives them a mechanical benefit.
Wizards might not care about reaction rolls, but they get an effective bump to intelligence when in their towers with access to their libraries, meaning they have a better chance at learning spells, and can cast higher level spells when they are at home. Clerics may be interested in the reaction bonus, but may also get similar spellcasting advantages to the wizards from their congregations.
Rogues get improvements to reactions rolls from a combination of fear and respect in the neighborhoods they control.
Mass combat is simplified by treating a group of men at arms as a single creature. For instance, a group of 10 peasant militia worth 13 XP each are written up as a Large (15' x 15') 4 HD monster with 18 hp and worth 132 XP. They might also have certain other special features, which I can expand on if this idea appeals to anyone. Make the units big enough and you can have groups of solders interacting directly with individual PCs on the same battlefield, using the same rules, and you don't have to have roll a couple of dozen attacks for all the 0 level attacks against the fighter, and all the fighter's attacks on the 0 level soldiers.
All of the above I have been thinking about for a while. From this point on I am spitballing.
Experience is granted for any scheme that the PCs pull off, or for improving your status. A lesser amount of experience is gained if NPCs pull off a scheme concocted by the PCs.
Players can run their NPCs, but the NPCs may have to check morale, and fall under DM control in the event of a failure.
EDIT: if you have a skill system there are other things you can do with this. For example, the success of an attempted action can depend on the skills or abilities of the PC or NPC running the op. This creates a market for the best steward, spymaster, guard captain, etc., and creates a trade off since the PCs can't do everything, but they are usually higher level and have better abilities and can bring their creativity into play so they aren't just relying on the numbers. This doesn't work as well with a roll-under system, where level is irrelevant to skill, or to a system that uses neither skills nor ability checks.
You can use something like the Assassins Spying table, and apply it to activities other than spying. That table is relevant to level but gives no advantage to a smart NPC spy, or a charismatic NPC ambassador, or a reeve who is a druid or ranger instead of a fighter.