It's probably a sandbox/underground hex crawl campaign, with a small area of caves making up the village and the well known immediate surroundings and fungus farming & fishing cave streams being the main food supplies of the village at the start. They'll have fuzzier knowledge and maybe some artifact maps of places up to a couple of days travel down tunnels and through caverns, and know some local dangers with strategies passed down for coping with them.
They'll be neolithically equipped, not a metal culture, with some shamanistic use of specialized fungi (& creatures? & minerals?) along with the usual eating kind. Spirit/ancestor magic seems a likely second magical path for them. The spirit magic could also include some attunement to the spirits of the stone - so they can appease elementals that might otherwise bring down the ceiling on them, etc. Memory songs - their records of the past would likely be largely retained in song. Their "bards" would also be keepers of tribal history. Re metals, finishing the thought, coming across metal weapons and tools will be a major and precious discovery. For this to be the case, the first few layers of opposition have to be unarmed creatures or similar to themselves or they would at least be culturally aware of metals.
Beyond that a cave network extends beyond their ken, much like the TSR Underdark.
Most of the tribe is very cautious about going too far, and after a coming of age test/hunt versus some of the local dangers and creatures, settles into regular patterns of life and ritual.
Possible hooks: Our PCs could be
a) an unusually curious generation, for reasons to be developed
b) thrust into the role of explorers by some creeping doom that imperils their village which can only be answered by finding a solution or place of refuge beyond the known extent of caverns
c) cut off from the village by a calamity (earthquake and rockfall maybe? shifting of an underground river?) during a coming of age hunt and looking for a way back the long way round, getting into adventures and exploration while seeking a way home.
Big discoveries possible for the PCs:
Strange or legendary creatures
More advanced worked stone
Subterranean places that are less of a forgotten backwater than their home
Magic, beyond the shamanism they have at the beginning
Other races & subterranean cultures, friendly or otherwise
Access to the surface
Rules? probably something OSR, but maybe Savage Worlds, Dungeon World, BRP
So does this appeal? Has something close to this been published in game modules I don't know? I'm not too familiar with the Underdark stuff, it being from the era between OD&D and 3rd edition when I got back in but think it was mainly from a traditional D&D surface explorers or considerably more advanced Drow perspective.