TL;DR: I want a D&D FPS with instanced player-built guildhalls you can fortify and assault.
My idea a while back was to have a shared world but with tons of available land parcels, and you can only own a parcel if you're on a premium account. So you can play for free and be a wandering adventurer, but if you want a shop or house in town you have to acquire a specific parcel of land. If you want a second house, you need a second premium account. You don't have to pay extra real money for this land. Just playing on an account that you have to pay for every month gives you the opportunity, and you have to spend in-game money and resources to make it happen.
Eventually I hope you'd start seeing market districts emerge, but probably you'd just have every single player try to have a shop on the ground floor, machinery in the back rooms, and a home above. How to avoid the whole city becoming a homogeneous mess? Maybe districts where the parcels don't allow machinery or NPC vendors (residential), one where parcels just don't allow vendors (industrial), and some which don't allow machinery (commercial). Certain parcels that are out-of-norm (like a cluster of commercial parcels in a residential district) would be much more expensive.
Anybody can come into your house. You need to invest in locks and stuff to keep people out. Picking locks is a criminal activity, so if it's witnessed the burglar may be arrested. But it's a choice, so if there's three burglars they don't automatically rat each other out. Also the chance that someone is passing by right then is going to be affected by how busy the district is, time of day, and number of NPC guards wandering around. Guards in a district are paid for by taxes paid by the citizens. If you don't wanna pay, that's fine, but you're more likely to get burgled.
You can use a command to eject a person from your house, but if they refuse it's considered trespassing and they can be arrested for it. This isn't a way for a homeowner to troll visitors and criminalize them, because they can just accept the kick and reappear outside.
You can add traps and vaults and stuff to your house. But unless you turn your whole house into a trapped vault you're probably better off putting your wealth in a bank, which would need to be set up by a player piece by piece using a commercial space and a vendor, or perhaps machinery that retrieves and stores deposit boxes. If you want your stuff back you need to go to the same bank. If the banking player steals your stuff I'm not sure that there's anything you can do - you're really just handing your wealth over to him. Pick a bank you trust!
The second part was "apartment-building" style instances like in Anarchy Online. For example, if you go into the mountains there's a cave opening which gives you a destination popup: it's a list of the areas in the cave network which might be underground lakes or a passage to a surface valley enclosed by mountains. A guild can acquire one of these to build stuff in. In order to acquire a guild fort you need a minimum number of premium accounts as members, and while a premium account can have characters in multiple guilds only one will count as a premium guildmember. A small guild fort instance may only require 5 premium guild members, but the largest might need 50. Again, you need to pay in-game money and resources to develop these locales by building facilities and stuff.
Finally you can invite attack by listing your guild locale as PvP, which encourages people from other guilds to raid you and take on your defenses. You can also set it to advertise attack by people unaffiliated with any guilds, or just premium loners. Keeping out the free players is mostly just an anti-trolling measure.
Your defenses would include traps, mazes, guard monsters, secret doors, magical barriers and teleporters and confusing stuff, hired NPC troops, and guildmembers. All this stuff would be built by the efforts of the guildmembers.
Your base number of NPC guards is based on the number of premium members you have. The NPC guards would spawn in at a barracks nearest the intruders, grab arms and armor from the nearest armory, and move to follow location-based orders you gave (manning siege engines or guarding chokepoints, etc). Injured guards who go too long without healing or enemy contact may flee to your infirmary to heal up and then return to the fight. Slain guards respawn soon but must reacquire gear. Attackers can strategically cut off or steal supplies from the armories and infirmaries. A stealthy attacker could injure a guard and lie low until he leaves his post to heal up, then continue sneaking in.
Looting the place can be problematic depending on how well you hid your treasury vs. keeping valuables in plain view. Did you set up a fake vault with decent loot but hide the best stuff elsewhere? And when they breach your vault to find that you store all your money in Copper Pieces, and it would take them hours realtime to haul it out in multiple trips? Then again, if you ever need to use that money it's hard for YOU to move it. Interesting choices all around.
Then you add special artifacts that are meant to be fought over. Magic spells can detect the location - narrowing it down to "this parcel" or "this fortress" (and then once you get inside you can narrow it down to a building in the fortress instance). If you possess the artifact you get cool benefits, but anyone can enter to assault your fortress regardless of your PvP settings. It's your choice to keep the artifact or not. This encourages top-tier players who want the best stuff to open themselves up to attack, which means their positions of wealth and power are insecure.
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How is this different from Haven and Hearth? Mainly that you can have an arbitrary number of guild locales accessible from one point at a cave entrance. And until those locales are bought by guilds, players can wander through them and enjoy the natural beauty, gather moss from a mountain valley, fish in an underground river. These locales can be randomly, procedurally generated so there's always, say, three of each type that are unclaimed.
Yet you still have hub towns where people can gather and live separate from the guild activities. Your house in town is part of a larger community, whereas the guild locale is a new community standing on its own.
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How to keep high-level players from raiding low-level guilds? I believe the difference in power between a beginning character and a solid casual character should be no more than 1:4, and the difference between a beginner and a maxed-out minus rares should be no more than 1:6, and a beginner vs. maxed-out-with-rares should be 1:8. That is, assuming player skill is equal, if eight 1st levels gang up on an elite max-level, it's a 50:50 chance on which side will win that fight. Diminishing returns on grinding, which helps discourage grinding. And makes player skill much more important - the strategy of which fights to take and how to prepare for them, the tactics of what to do during the fight, the strategy of recovering from the fight and returning to safety.
In this way, let's say you have a guild of 10 premiums, none of whom are there to defend their guild valley from an assault by 10 maxed-no-rares. Those NPC guards are only going to be equivalent to a beginner, but they have siege weapons and can focus down on one attacker at a time. They also respawn, which means the attackers will probably end up fighting several dozen guards in total. There are also traps, which helps wear down the attackers. But the attackers are effectively bringing 60 man-equivalents, while the guards bring 10 man-equivalents. The attackers will probably win the initial assault.
The attackers use magic to locate the big concentration of loot - probably the vault. They search around, but must stay together else the respawning guards will gank a loner. Thus the search takes some time, but eventually they find the unmarked door which has a big vault door behind it. Perhaps the big trap which was placed here will completely kill one of them. In any case, while some work on the vault, the rest must defend against the guards' assault on them. And the vault has been designed to offer no cover for anyone breaking in ...
Finally they get the vault open. They're probably running low on magic, on healing, and the guards keep respawning. By now they've probably fought well over 60 guards. The vault is open and they can loot, but looting takes some time. They need to haul the loot out and while carrying the heavy bags they can't fight effectively. The haulers need to be defended by the other assaulters while they escape. Now we have a fight of 10 guards vs. only half the assault team, so it's 10 man-equivalents vs. 30. The outcome is not so certain!
Assuming the armory hasn't run dry the guards can keep spawning and grabbing. The assault team could split in two and have half cover the armory or spawn-camp the barracks, while the other half handles the vault. But that means it's 10 unequipped guards vs. 5 attackers, so perhaps 5 man-equiv vs. 30. A smart guild will build two exits from the barracks, each passing through an armory - meaning the spawncamp squad needs to cover multiple exits. The spawn squad could haul all the weapons out of the barracks into a hallway behind them, but that is going to take some time. This isn't a game where you click to pick up a stack of 256 swords and put it in your backpack and walk away.
Perhaps a thief could sneak in and haul everything out of the infirmary and drop it in a corner somewhere, lay bear traps in the halls leading from the barracks, scout the place, and generally just set up for the assault while the rest of the team waits. Maybe you could have an assault team of ninjas just stealthing through it - although with greater stealth skill comes less weapon skill, and they're wearing light equipment and armor, so if they're rumbled the guards will chew through them much more easily.
As an ultimate defense, the guild probably just doesn't have so much cash on hand. They spend their money and resources to build, which is very difficult and time-consuming for attackers to tear down. The guildmembers keep some cash on their persons, some in their homes in town, some in the guild valley.