I can’t remember where I read it, but it was on a blog in the past few days. The idea was to have your “name-level” PC get a domain (stronghold, village, whatever) that would “attack” nearby resources to gather them. So it has HD and uses the same sort of systems as in the main game.
I’d like to expand that idea to just have a character sheet for your domain. HD is population (in general units of people, not individuals), AC is its defenses. You could utilize the six ability scores too (CHA affects immigration and trade).
Another way to look at it is like board games that have a central board and each player has a side board where they develop their own stuff. The central board has limited opportunities that the players scramble for and bring to their side boards. I would say the domain is a personal board and the player develops it as a side interest, with the PCs coming together to adventure in the “main board” of the rest of the game setting. Of course, players would be able to affect each other’s domains if they wanted, whether by taking central resources or directly assisting or hindering each other.
Each class should utilize different resources more effectively, should gather some more efficiently, and those two shouldn’t necessarily be the same resources (encouraging trade). They should have different goals too, for example an M-U might need to have a domain to create spells, magic items, dabble in cloning, etc. Clerics might be able to create holy relics, healing potions, holy water, train Paladins, etc. Fighters would have the largest forces of trained warriors, and thus able to protect the largest amount of land and gain the greatest agricultural production (needed to feed those men … ). And all of this should be graphically clear with the layout of the personal domain boards exactly as it happens in the board games that inspired this.
Basically, I’m trying to encourage the D&D endgame, partly by making domains feed benefits into the PC’s adventuring, partly by not requiring retirement to run a domain (such as “taking care of your Thieves’ Guild takes up all your time, no time for adventure”). If a PC enters the domain game late, it’s not a huge problem, since they’re not in direct competition and the benefits of adventuring still outweigh the income from the domain. In fact, I look at the domain’s profits as going into domain development instead of the PC’s purse, and the development allows transactions instead of giving direct benefits.
A specific example would be an M-U who wants to clone up some monsters to assist him. He needs a secure place far from the prying eyes and pitchforks of the locals: a fortified tower or mansion would be ideal. After setting up whatever defenses he feels necessary, he must spend resources to set up the vats and apparatus for monster-mashing. All this requires resources to be gathered from the countryside: weird plants, monster parts, bottled gasses, salts and sulfurs gathered from steaming pits, etc. And of course craftsmen need to make things. So the M-U either needs to gather villagers who can gather these things, or needs to have magical minions do it, or do it himself. However he manages, all those resources don’t go toward the “GP” line on his character sheet. Instead he now has his vats, and can spend in-between-adventures time mixing up the poor things. His future resource gathering will probably go toward a spell research library, an alchemy lab, observatory, more defenses, improved resource gathering, a special forge for working adamantite, crystal gardens, etc. His vats will produce some monsters that he can bring on the adventure or use for defense of his mansion.
If his M-U buddy doesn’t want to do the same kind of domain side-game, that’s fine. He will end up with an excess of money: he won’t be able to spend it on making magic items, he won’t be able to spend money to grow weird crystals, etc. Maybe he can make a deal with his buddy who has a domain, to buy his stuff, and maybe his buddy will sell him stuff at-cost. Or maybe he will decide to make a domain of his own and develop different things, and the two M-Us happily trade their wares.