I doubt Kratos will be around for Ragnarok. Reason being, Atreus / Loki is already confirmed to have a time-traveling son in game (Jormungand, the World Serpent), and Jormungand fights in Ragnarok (confirmed through Mimir, if I remember correctly), so Loki has to be an adult by the time it happens. If the Jotun mural is to be believed, Kratos dies when Loki is still a boy. Maybe even at the very beginning of the next game. So, my theory is this:
In the next game, Kratos dies early on. His godhood / rage / whatever is absorbed into Loki, who becomes a fully-fledged god and is approached by the Norse pantheon (I really think they want his powers because of Faye, idk why specifically, but alas). Shit happens, years pass, Loki becomes a man, gets with his giant wife as told by the myths, has kids. Fenrir, the wolf, Jormungand, the World Serpent, and maybe Hel, goddess of the dead, if they're feeling spicy. All participate in Ragnarok, and from there, idk what would happen. I don't think they'd send Loki to like, Egypt or something, but I could be wrong. I can see them doing a new protagonist completely.