Some job listings for Horizon Forbidden West developer Guerilla Games have been spotted and they may be pointing at multiplayer, co-op, or something entirely different coming down the line.
They're looking for a senior social systems designer "with a passion for player communities and social features" and knowledge of ways and systems of how players can connect in games. This designer is also expected to know how to "engage players in social interactions to create lasting relationships," with the goal of creating guilds or exploration groups together.
The other listing is for a senior games writer with "extensive knowledge of stories and narrative design in open-world RPG games, online games, and MMORPGS". This could be for the same project or something different. With Horizon already taking place in wide open worlds ripe for exploration, with deep lore, and memorable creatures, there is definitely room for something social and multiplayer to happen.
According to VGC, which first published speculation on the job listings, Horizon Zero Dawn concept art that was leaked in 2014 showed a potential multiplayer mode where players could battle some of that game's creatures. There's also word that Sony wanted Guerilla Games to include a multiplayer component in Horizon Forbidden West, but the team decided to concentrate on development and save any multiplayer for the future, perhaps for a spinoff. With Horizon a flagship franchise for Sony, it makes sense that they would be looking to expand and possibly add engaging repeatable multiplayer content. Of course, these hires could be for a completely different project entirely.
Whatever is in development, and this is the earliest of speculation, Horizon Forbidden West is coming in just a few months, so it's no surprise the dev team is likely looking towards the future. That future just looks like it might contain some kind of collaborative or social multiplayer in the Horizon universe or another.