First, we could use the
RP map of Wintreath and carve it up into a bunch of tiny provinces like in Medieval Risk. Then, each person could own one province (calling back to that
citizen=freeholder discussion), but familial houses could own each of its members' provinces plus extra (as the sum is greater than the individual parts, plus to encourage joining/making a house). Perhaps Common Houses would have to pledge loyalty to a Noble House (aka make an alliance), but citizens who are unaffiliated would essentially be under the Royal House until they decided to join/make a house. The Royal House (headed by Wintermoot, of course) could either be an active player in this or a passive "everything not owned by a house is owned by the Monarchy" (or both) and would have ultimate authority (as this
is a Monarchy, though the Storting would deal with the day-to-day).
Each province would earn a player certain resources each day/week/whatever (e.g. currency, food, troops, ships, arms, wood, stone, metal, etc., depending on how complex we want to make it), which would likely be tied to where it is on the map (e.g. provinces in the North aren't good at producing food, provinces with forests in them are able to produce wood, etc.). Each house could have a specialty to give a small bonus (e.g. House Burdock could have an Agriculture Specialty, so they produce extra food in provinces). Houses and Citizens could then trade/expand borders/battle each other. Then at the end of a certain timeframe (perhaps every three months to tie to the seasons), the game would reset, but the houses that had the most resource/provinces/whatever could have bonus starting resources in the next term. Maybe each season would give certain bonuses (e.g. extra food production in the autumn, attacks are more difficult in the winter unless your house is from the North).
And for players who don't really care to take part (whether that means ever or just for a couple weeks because of IRL stuff), they can just continue to own a province, which continues to produce resources to the Royal House (if unaffiliated) or their house (if in one), and not play. I'd imagine this game would not need to be as intensive as one of our regular forum games since resources would passively grow for everyone and turns would be the same length -- but you can be as involved as you want in the diplomacy/trading/war aspect of it or just idly let your resources grow.