I was going to suggest this as a setting over in @orin's
pitch thread, then I realized it'd probably lead to a derail, so...
Did you know there don't seem to be any Hollow Earth MUDs? It may be a totally unexplored setting. For those of you unfamiliar with the concept, the idea is that below the surface of the Earth, there may be massive cavernous pockets of space that are inhabited by humans and/or fauna and flora incognita.
Name: pendingCore Theme: Region vs. Region vs. Invaders
Sub Theme: Politics, Trade, Exploration, Steampunk
Setting: The game takes place in an undefined era, in a Hollow Earth setting. The map is divided by a rolling ocean connected to the outside world, but unable to flood the dome because of limited points of entry and no way to displace the air trapped inside of it. The ocean is teaming with life, some of which completely unknown to the outside world. The setting has a constant dim green light provided by phosphorescent lichen growing along the dome. This lichen, along with seaweed, provide fresh air to the dome. Stone and metal are commonplace, but wood and fuel are more difficult to come by. Power is generated using geothermal wells on land, and is either used immediately or for charging batteries.
Beyond this underground setting is an undetermined surface world nation that has discovered it. The surface nation has sent agents to infiltrate the underground nations, but their true intentions are unknown. It is highly suspected they are planning an invasion. These potential surface world invaders are the only ones with submarine technology and can only access the Hollow Earth by submarine. Their entry points are unknown.
Location: The game takes place primarily on land, as the continental factions and archipelago compete for resources and influence. The land is distributed between 3 of "continents" alongside the edges of the dome, a single continent surrounded by ocean, an archipelago nation, and various independent islands. Civilizations with access to a dome wall or a (natural) support spire tend to build into it or along it, to maximize how much flat land they have available to for crops, animals, or "wood" farming. The game also takes place at sea, but there is no escape from the Hollow Earth.
Feature List: Crafting, exploration, research, PvP, RvR, trade, unusual setting
Playstyle: Highly varied. Depending on how many players were around, there'd be a lot of room for people to involve themselves in politics, piracy, trade, exploration, selling out their underground brethren to the invaders, etc.