So, I just had an interesting idea. Completely unrelated to my current project but I thought I'd share it in case it was of benefit to someone else. The idea is a game where each player has their own section of the "world" that is theirs alone. They can add friends who can visit this area, or travel to random areas that may or may not have other players.
I framed an implementation as Space Minecraft, which the player can build stuff in space as well as build their own spacecraft in the familiar minecraft way. Each player has their own "zone" of space which is filled with asteroids that they can disassemble and use for construction. They can make friends and travel to their zones for visiting or whatnot. An guild could even could set up a zone as a space port for their members.
Griefing is prevented by only letting players travel to zones that they have been given permission to travel to.
Players who need more raw materials could travel to a random zone for harvesting, which may or may not have other players in them. PvP would be a logical consequence of this.
The server for each zone would be the computer belonging to the zone's owner (random zones would be served by the developer), which should drastically reduce maintenance costs.
Furthermore, players could play offline without the ability to have friends visit or to visit friends. Likewise the random zones they traveled to would not have anyone else occupying them, and would probably have reduced resources as well. (to reward players for chancing PvP online).
Might be tricky to implement, but I think it could pay off in lower costs for running the game.
I supposed you could kind of view this as a peer-to-peer mmo.