Appearantly, the rumors about a new, unannounced Horizon game were true, and they are not just rumours anymore. A while ago, Guerrilla Games announced that they were working on three new projects, including Horizon Forbidden West. And now there is only one project left that we don't know anything about.
According to Guerrilla Games' tweet, HCM will be a PSVR2 game that uses the new heaadset and controllers. The protoganist is still unknown but it's said that we will encounter some people from HZD and HFW, including Aloy. Guerrilla also said "with the hardware's new controllers and incredible built-in visuals, Horizon Call of the Mountain will allow you to interact with the world around you in a brand new way."
Also according to Firesprite, they are working with Guerrilla in this project.
I cannot wait to see the result!