Upcoming Heroic Fantasy Companion got me thinking about Arthurian myth, which in turn led back to Pendragon. It's a horribly baroque system, but one interesting thing that it does is have players start with manors - very small domains. AskHistorians puts the population of an English medieval manor at around 300, or 60 families, which under the most recent revision of the ACKS domain rules generate 300gp/mo in revenue in civilized circumstances (low garrison, but tribute to liege lord). They also occupy about a quarter of a six-mile hex (at default "initial" civilized population density of 280 families per hex or Medieval England's population density of 250 families per hex), so you could fit four manors per hex... which is roughly one per PC in your average party. At 300gp/mo in revenue, you would hit 4th level before capping out on your domain threshold. Such a manor requires a "fortress" worth 4000 gp to secure - a large stone house with some palisade walls ought to do the job.
Depending on how much ACKS domain detail you want to preserve, you could then go and do the whole percent-increase-per-month-adventuring and domain morale and all that... or you could take the Pendragon approach, which is that a manor is a manor and it gives you some static income (maybe modified by a roll), and then you make manors your basic unit of land-tracking (with settlements handled separately). Pick up more civilized manors by marrying rich, conquering some Saxons, or getting a land grant from the king for services rendered (a hard thing to do IRL, but an exceedingly reasonable thing to do from an RPG perspective).
An interesting property of the domain XP system that we really haven't gotten to experience is that it's almost a second form of reserve XP - if you die, your heir will (slowly) level up back to some hopefully-reasonable level on domain income. "Slowly", of course, is the operative word - first to second level with a single manor is ~8 months for a fighter with no prime req. Pendragon's dynastic pace (something like one adventure per year of game-time, and I would imagine about one year per session) could address this as well. On the one hand, this creates problems with saving money and maintaining armies, but on the other hand, it also opens up a lot of room to actually get some use out of ACKS' research, hijinks, construction, troop training, aging, &c rules, which are usually squashed out by "always be adventuring".
But at some point we're really just looking at Pendragon with ACKS' combat, proficiencies, and market systems. Hmm.
In any case, I do think that "start players with small, civilized domains and a connection to a liege of some sort" is not a bad idea. Passive income, assisted recovery after PC death, and connection to gameworld are all good things in reasonable quantities (particularly as I swing backwards on the sandbox pendulum, towards "you know quests could actually help alleviate some of the emergent tyranny of structurelessness / informal hierarchy that we see in sandbox play").