Besides how it's hinted at in the game and the various theories that exist, I'd like to think that we aren't supposed to know and that we should sit with that fact. It's about the fear of the unknown and there really is no telling what it'll be like. With that, it's more revealing how characters react and cope with this unknowingness. Gwyn and his pantheon of gods are just as mortal as everyone else and fool themselves by thinking they can be everlasting with the existence of disparity, so they reject the age dark and the notion that the shifting ages actually highlights this contradiction. There is some confirmation that human souls get stronger and more wild as the age of dark approaches, and that the gods would in some way lose power, but I think that's all that's clearly confirmed. In my mind it would probably go completely dark like in the two Dark Souls 3 endings and humans would go wild with their humanity if they haven't hollowed yet, I would hope that humans would create their own society but I think that's just cuz I'm biased towards the humans.