This is my next campaign idea I'm working on, to be run over on odd74 forum. Curious 1. your feedback/comments, if it sounds fun or maybe you have a suggestion to make it better; and 2. if you are interested in playtesting any of these concepts in our ongoing Hobbet Heroes campaign?
Mushgnome's Underworld Survival
When I was a kid, one of my favorite books was the AD&D
Dungeoneer's Survival Guide by Douglas Niles. I never actually got to use this book with my friends (because everybody wanted to play hack-and-slash one-shot adventures) but I spent many hours flipping through the book and studying the maps. I dreamed of someday finding the right players for an underground 'point-crawl' campaign, where the PCs would cover vast distances underground, interacting with alien civilizations, and going weeks/months/years without seeing the sun.
Mushgnome's Underworld Survival is my grown-up version that dream. But rather than using the DSG's implied AD&D setting (which has way too many MM2 monsters, like cloakers and aboleth, for my taste) this will be 1974 style: based on the OD&D '3LBBs' plus Outdoor Survival. You know how a lot of D&D players use the Outdoor Survival map as their campaign 'wilderness'? This campaign is kind of like an underground version of that, with the PCs encountering various underground hazards as they canvas the map looking for resources (and trying not to get lost).
The 'resource management' portion of the game tracks 5 main resources: food, water, light, each PCs' individual hit points, and the group's overall Life Level (a concept from Outdoor Survival). The party's Life Level is interactive with the individual PCs' condition. For example the party can trade a Life Level to heal a wounded PC. The PC regains their lost hit points, but the party travels a little bit slower, representing the burden of caring for the wounded member. Or an example in the other direction, if a fatigued party is joined by fresh reinforcements, this might restore a Life Level, to represent the vigor of the newcomers. (Note: The number 0-6 on the life level index is the number of hexes/day that the party can travel. If the number drops to 0 then the party is stranded without resources--you'd better hope a wandering monster comes along, so you can kill it and eat it!)
In contrast to Hobbet Heroes (where most of the monsters encountered have been animal intelligence), Underworld Survival features a large number of intelligent underground civilizations and 'factions.' There are many opportunities for diplomacy and trade. We have some house rules for languages: Instead of a set list of languages, each faction speaks its own dialect, and PCs can attempt communication on a successful INT check. If they succeed then they get a score of 1 in the new language, and this skill can improve over time as the PC continues to interact with that civilization. Theoretically a PC could learn an infinite number of languages, if they spend enough time in the underworld.
Another core house rule is that I will be awarding experience points for economic activity. For example if the PCs establish a trade route, or dig a mine, or tap an oil well, I would definitely award xp for that. If the campaign runs long enough, for the PCs to reach domain/name level, it would be a given that they build their strongholds underground and rule over an underworld domain.
The campaign begins in a tavern on the surface. The PCs are surface-dwelling humans/dwarves/elves/halflings who are teaming up to investigate a treasure map. It's a strange map because, instead of showing north/south/east/west, it shows up and down. The map shows the major landmarks within 2 or 3 days' travel straight down. Below that, the players are on their own!