The "God of War" easter egg is actually its own side quest in “Horizon Forbidden West,” where Aloy has to gather "God of War" themed totems. The Kratos totem sits on top of a mountain, southwest of The Bulwark.
Located in the open world is an unkillable Inari fox, similar to the ones found in "Ghost of Tsushima." Just like those foxes, if Aloy follows the Inari all the way through its path, she will find a "Ghost of Tsushima" shrine.
Patrick Munnik was a lead producer at Guerrilla Games for eight years, before he passed away at the age of 44 in 2019. Players can find a memorial dedicated to him on an island on the western edge of the map.
Located in a secluded part of the map, there is a massive stone with a sword shoved into it that players cannot interact with in any way. While video game fans will want to think it's a "Legend of Zelda" reference, it seems like it might be a reference to the legend of King Arthur.