A recent teaser in Ubisoft's The Division 2 seemingly confirms that the next Assassin's Creed, set to release in 2020 and codenamed Assassin's Creed Kingdom, will feature Vikings. And because of this, it seems that God of War director Cory Barlog may want to get Kratos in on the action, due to the two games' Norse connections.
It's worth highlighting the serious difference between the two titles before moving forward. God of War is set in a mythological background adhering to Norse belief, but the game itself does not feature Vikings, nor is it set in the era of the Viking. Assuming that leaks concerning Assassin's Creed Kingdom have been thus far accurate, then we can expect our new protagonist(s) to be of Viking heritage and set in this era around 1000 AD, hundreds of years after Assassin's Creed Odyssey.
Although these games have serious, if hard to detail, differences, a crossover would not be thematically out of the question. For example, it could be something as simple as seeing Kratos depicted alongside other Norse gods or as complex as receiving some God of War-themed gear in Assassin's Creed Kingdom (though if something like this did come to fruition, it would likely only be available to PS4 players).
Of course, because Barlog's statement is given in a tongue-in-cheek manner, this could amount to nothing, but crossovers have occurred because of Twitter exchanges before. This move would then have precedent for Assassin's Creed Kingdom, and it's worth mentioning that God of War wouldn't be the first franchise crossover in the Assassin's Creed storyline, as Assassin's Creed Origins features a special quest related to Final Fantasy 15.
Considering God of War was one of the top 10 games of 2018, a crossover would likely be welcomed by fans, even if it's just to briefly enjoy more Kratos-or Atreus- related content before the next game in the series.
Assassin's Creed Kingdom is rumored to be in development for unspecified platforms.