Sony has already hinted heavily that Kratos's time is finite. You only need to review the ending to the game to see they already have a "destiny" set for him. As such, I believe the answer is that the God of War series will branch off to be about Loki.
Regarding mythology...Romans built upon Greeks. It'd be stupid to have that as a progression forward, because it'd be a retread.
Celtic lore has plenty of monsters...but not a lot of god interactions.
Anglo Saxons have the same issue, with paganism being the problem.
Etruscans have the Roman issue and animism...which is hard to care about.
Japanese is not mythology. Shinto religions focused on Kami exist. Not a bad thing...but the animism is still going to be silly to convey.
I've pooped over everything...then what's my response? Egyptian, specifically ancient egyptians. Thing Prince of Persia. If not that, Hinduism. Very strong mythology, polytheism, and clear gods. There's literally zero motivation for anyone in the industry to move on to subsequent religions...and they've already hinted that the future might hold the Mayans or other central and southern american religions. Either way, retreading the Greeks via the Romans with the new play style would be unlikely to sit well with the core audience, given the stigma most remakes get now that agenda is often prioritized above making a fun game.
I see two to three Loki focused games, a return to Kratos who has been somehow reborn when they burn whatever they needed to explore there, and moving towards more obscure but fun religions. So we are clear, I state the above as fun because from the western audience perspective Europe and Greece aren't new, but seeing Vishnu and Tyr tear it up would be fun.
