Despite his relatively short screentime, Rost is one of the most important characters in Horizon: Zero Dawn. As Aloy's adopted father, he attempts to steer her in the right direction and teaches her how to survive from a young age. Rost unfortunately meets his fate and is buried outside his hut during the first game, making his appearance in Horizon: Forbidden West initially seem unlikely. But that may not be the exact case.
Right now, there's not a lot known about Horizon: Forbidden West, other than a few details about where the Forbidden West actually is. Aloy will be traveling across parts of the United States once known as the Yosemite Valley and California. This is notable because Rost and the Forbidden West actually share some history.
Rost - Before Aloy
Prior to taking on Aloy as his ward and daughter, Rost had a family. He and his mate, as the official lore puts it, had one daughter, Alana. She didn't get to experience much of life, however, as she was killed by a band of outlanders around the age of 6. Grief-stricken, Rost sought the High Matriarchs in order to be made a Death-Seeker, someone who was consider ritually dead. He set out to avenge his family, never to return, except he did.
He managed to slay all those who took his daughter from him, crossing through the Forbidden West in the process. As a Death-Seeker, he wasn't supposed to live through the mission but did. Upon his return to the tribe, as he wanted to die as close to his home as possible, the Matriarchs cut a deal: they would allow Rost to live outside the village as a Nora outcast, keeping as much of his mission as he could a secret, and he gladly took them up on this offer.
Eventually, he would be asked to raise Aloy and gladly did so. Aloy, in so many ways, was the daughter he lost, and even during his brief screentime, Rost makes a name for his self as a video game dad. Rost would eventually give his life to save Aloy, though, meaning he finally met his fate as a Death-Seeker.
Rost's Death and Potential Appearance in Horizon: Forbidden West
Player shouldn't expect a Resident Evil-style revival of his character, as he was buried, and Rost's death seems final. However, Aloy will be presumably traversing the same parts of the Forbidden West as Rost. What this means is that Rost may be able to posthumously aid Aloy, while pulling at the heartstrings of the player, through her Focus. It is capable of a number of things such as scanning missions, analyzing a machine's trail, and spotting trails left by humans. Furthermore, datapoints and glyphs could be de-coded by the Focus, meaning if Rost ever past through any areas where perhaps he shouldn't have as a Nora, there could be recordings of him left behind.
Even if it required expanding on the lore a little bit as it has with new machines like the Sunwing, Aloy's Focus could act as a connection to the past and reveal parts of Rost's journey through the Forbidden West. How and to what capacity remains to be seen, but having Aloy and Rost in the same area without there being some connection would seem like a wasted opportunity.
Horizon: Forbidden West will launch sometime in 2021 for PS5.