Basically a building game, maybe voxel maybe not, where aesthetics were rewarded and you had a reason to build.
I imagined it like this: Land of Lords is a terrible game, BUT it gets one thing right: putting the players on the map. Literally, but also figuratively. When you build a settlement of significant size in this new, fictitious game you get a dot on the map. As you grow from hamlet -> village -> town -> city, add fortifications, castles, etc. etc. you get more territory that is represented. In-game power as represented by both titular titles and the ability to conduct warfare, bring other domains under your influence, and more efficient resource gather is bestowed upon you. Aesthetically, the end goal is increase your urbanization to the point where you can actually see the extents of your city represented on the map. At which point, you get further rewards via player voting on the server's/game's most awesome cities.
Likewise, your territory is also better protected, you get guards, miltias, standing armies that must be defeated to inflict actual physical damage on the domain. This part is a little tricky to balance out, but could work simply by calculating the attacking and opposing force totals (with force multipliers in effect). As you get a larger domain, it is exponentially more difficult to attack and damage your stuff. At one house you have no protection. At a hamlet you have a guard or two that must be defeated before you can wreck shit and loot stuff unless there is a significantly larger force of bandits or soldiers attacking. At city, you have several layers of protection including the guardsmen, militia, and standing army which must be defeated before the city can be sacked. Walls are always able to destroyed as to facilitate vigorous and purposeful siege warfare with an emphasis on breaching the city, killing the inhabitants, and then taking control without having to erase a players life work. If a player is totally defeated ya, you can raze his city to the ground.
Speaking of which, quick segue into devastation and abandonment mechanics. Players leave the game--that's a given, so based on activity, growth, and negative events such as banditry and warfare, settlements may be marked as abandoned and slowly rot away at which point they are taken off the map. Their remaining populations migrating to adjacent areas or having a smaller chance of perishing in the wilderness. Likewise, if there are substantial areas of unclaimed land, neutral NPC settlements may pop up. These are mostly a good source for banditry, low-risk low-reward endemic warfare, and interesting tales/RP. Most of these kinds of settlements cannot grow very much. Occasionally there will be neutral cities and castles to explore or to conduct diplomacy in. When building a NEW settlement it is important to note that based on the size of any player settlements around, you'll have to venture a certain distance into unclaimed land to begin building--neutral settlements will instead quickly begin to crumble as you expand and do have much territory to their name.
You'll always be able to build blueprints in advance to get started so that you can auto-construct things later. A system by which the environment will be altered as you build will have to be devised so we don't get any janky gross-looking Minecraft-esque disasters. Assuming it's a voxel build system--it doesn't have to be. But the idea is that you have a very large control over the aesthetics of your domain. Including buildings, clothing, ships, weapons, etc.
Warfare will also be a little tricky. Probably a combination of individual management for guards and Total War-esque system for the militia and standing armies for warfare purposes would be used. So you can still have guards walking around, but be able to maneuver large formations in battle. Balancing would be tough, but ideally different kinds of armies would be viable and would be affected by weather, terrain, and quality. Units get experience, can be customized, renamed, etc. etc. the work.
Finally, economics. Through some combination of open market trading, supply-and-demand, and physical trade routes would be conducted the process of buying and selling materials on a grand scale. Economic warfare would be a viable strategy in both the physical game area and through the use of embargoes.
It's not perfect, and errs very closely to the simulate-everything kind of game--but it'd be awesome.