Maybe you can discover the Diggers when you find their settlement, like their Admin Centre is the cave entrance. They could have tunnels to each other and other fun surprises. Would also be a great mining opportunity.
My vision for the diggers is to have them inhabit an entire underground world. There would be caves dotted along the world that lead to the cave "layer", which is effectively a second world map. By travelling underground you can exit through a cave in a different area on the surface.
It could add a neat dimension to trade and warfare, with armies appearing from caves potentially well inside your empire.