This sounds awesome!Dragonbane as the main rules set.
The BECMI rules for dungeon exploration, wilderness travel, and domain management.
Red Tide for generating borderland towns, courts, and small dungeons.
De Bellis Antiquitatis as a mass combat system.
A Foundry campaign with 10 to 18 players who each have one to three PCs (plus hirelings) that make up a mercenary company seeking fame and fortune in a borderland region.
Adventures are either scouting patrols into the ruins around the current base camp West Marches style, with whatever 3 to 6 players can make it that day, or long range expeditions that will resume whenever most of the players on it can all get together to play again. (Which is why regular players will probably need two PCs at least. It might be some time before an expedition can resume, but you can still play patrols.)
The setting is a sparsely inhabited world of forests and mountains that I imagine as "George Lucas and Jim Henson produced an AD&D movie in 1989 l that was shot in the Sierra Nevada". Society and towns are like 6th century medieval, but the animals in the primeval forests are all big reptiles, huge insects and other arthropods, flying lizards, and fierce rodents. The nonhuman peoples in the wilderness are bug-goblins, ogre-sized stone giants, graceful harpies, gnolls, and fish men.
Civilization is very small with just half a dozen city states in an area the size of central Europe, but there are several layers of ruins from various different civilizations, each one more inhuman as one goes deeper down in time. The environment and local climate is controlled by the spirits of the land, which makes it very unstable and unpredictable on larger time scales. Large areas become useless for farming or surges in new predators destroy lifestock herds every few years or decades somewhere in the region, and there is nothing the affected people can do but pack up their things and find a new place that has recently become suitable for farming and has not been settled yet, creating a permanent Migration Period environment. This does include the few larger cities, which have all been build on top of older ones, and there are many more that are currently abandoned ruins, but might become great city states again some centuries in the future. Because of the regular disruptions and lack of continuity, history more than four or five centuries back is everyone's guess. But it's believed that this has always been the way of the world, and always will be.
The gods are forces of nature first and never appear in physical form or make any contact with their priests. Religion is primarily about understanding the gods' effect on the world and adapting society to make the best out of it. Get out of the way of destructive forces, but learn to benefit from the opportunities created by their passing. Priests usually see the nature and workings of their gods as examples how one should deal with life and have developed moral and ethic philosophies about emulating their divine traits.
In addition to the gods and spirits that are the driving forces of the natural world, there also still exist the Primordials who predate the first appearance of light and fire. They are beings of a world that is only darkness and water. Since the appearance of the stars and the sun, they have retreated deep beneath the earth and the bottom of the oceans, or far out into the Void away from the heat and light of any sun. They exist completely outside the ecological system of the natural world, and even on a supernatural level they are almost completely different from spirits. The world beneath the sun and moon is lethally hostile to them, but the further one descends into the depths, the more traces of their continued existence remain. The primordials draw heavily from the D&D books Lords of Madness and The Forgotten Temple of Tharizdun, and the games Darkest Dungeon and Bloodborne.
I'm hoping to get this launched by early summer.