Here's one idea, based on the very loose model of domain management described by Gary Gygax in OD&D and AD&D1, but adapted to the 5e rules
What do the Players Do?
- Players clear the land of monsters
- players set up patrols to keep monsters away
- players build the castles/fortifications
- players collect income based strictly on number of inhabitants
- players may “invest” in the domain, building roads, farms, etc, but don't really have to. These should be able to be summed up in a page or two.
What do the DM's Do?
- It's up to the DMs what buildings and businesses the peasants build
- DM's come up with the threats and problems for the domain
- There is no chart of domain loyalty or the like, the DM says: here's a problem (landless laborers are leaving, will decrease population/income), and tell players to fix it (put squeeze on land holding peasants for higher wages, capture slaves, threaten those who leave).
The main thing to do is get a functional economy system working: need to square the Building and Maintenance costs from the DMG with some individual improvements and figure out what the income per inhabitant and advantages of individual improvements.
Using 5e rules, most of the building and mapping of the fortress design will fall on the DM's, unlike the previous editions, but let's face it, you and I would end up doing it all anyway.
The other thing we would need to handle would be underlings. 5e is just not focused on that. While “commoners” and “guards” should be common enough; how many “veterans” or “knights” would be available to hire? How much do hirelings get paid? Those are the kinds of problems.