PurposelyCryptic

... The whole point of the Norse GoW games was to show an older, wiser Kratos' experience a second chance to live a better life, one of stoicism instead of mindless rage and hate, in order to provide his son with the proper guidance he himself never received, and thereby allow him to avoid having to learn the hard way that a life filled with only anger, hatred and pain ultimately ultimately only leaves you empty and broken.

It was always intended to be a father-son experience, with Kratos doing his best to guide his son along the right path, himself learning how to do so along the way, as well as becoming a better man in the process, by seeing himself reflected in the frequent mistakes his son makes along the way despite Kratos' best efforts. They both learn and grow together, becoming better than they were.

Replacing Atreus with a female companion would have been an entirely different game, and, at the point where the first Norse game began, I would argue he was not yet ready to fight alongside someone else as an equal. If Fey had replaced Atreus, he would have just ended up not letting her fight to her proper potential, keeping her at his back and protecting her instead of acknowledging her as a partner on the battlefield, just as he ended up doing with Atreus; except Atreus was a child, and needed protection and guidance - Fey did not.

The next GoW game seems to be headed towards Egypt, and will likely feature Kratos together with Freya, with Atreus playing some sort of secondary role as he slowly learns to become his own man without his father constantly at his side to catch him if he falls. She already played a partner role regularly in Ragnarok, and the ending was heavily hinting that they would be traveling together going forward.

So we'll probably see exactly what you are asking for, Kratos with a strong woman, Freya, standing at his side, each supporting the other, with a strong established relationship forged from their past experiences together.

There are bigger issues, like the region being almost exclusively inhabited by Ainu during the time period they chose to set the game in, with only a few small Japanese settlements. And when the Japanese did begin to fully settle in the region, they treated the local Ainu about as well as European settlers treated the native American population, if not worse.

Ghost of Tsushima was a fictional story centered around an actual famous historical event, where you had an enemy force that was a clear and obvious aggressor: the real-world population of Tsushima was effectively wiped out entirely by the Mongol forces, so they could use it as a staging ground for the invasion of Japan proper - if a massive storm hadn't sunk much of their fleet, allowing Japanese forces to retake the island, who knows what might have happened.

Ghost of Yotei... Well, it doesn't have any of that, and is set in the completely wrong time period, towards the end of the Sengoku period, an almost 200-year period of near-constant war, in the first year of the Tokugawa shogunate, the official beginning of the Edo period. After the death of Oda Nobunaga, Toyotomi Hideyoshi finally unifying Japan and subsequently unsuccessfully invading Korea, and the Tokugawa clan overthrowing the Toyotomi clan after his death, no one was really looking to invade Ezogashima proper (modern Hokkaido), let alone Makkarinupuri (the mountain today known as Yotei-zan).

The Japanese had subjugated the ancestors of the Ainu, the Emishi, starting in the 9th century, and slowly pushed them out of northern Honshu into Ezogashima and the othern northern islands, but outside of some minor isolated settlements restricted to the Wajinchi region (literally, "Japanese people's land"), which covers the peninsula bordering the Tsugaru Strait separates the island from Honshu, there was no Japanese presence beyond trade activity in the rest of Ezo until far later, in the 19th century.