Guerilla Games is about to expand the Horizon universe next year with Horizon Call of the Mountain in February and Horizon Forbidden West: Burning Shores in April. In addition to this, Netflix is in the middle of developing a live-action adaptation of the franchise, which will reportedly explore the years that led to the catastrophic events that created the game's world. But, it appears this isn't the only Horizon-related projects fans can look forward to, as the studio is working on a "separate Online Project" set in the same universe.

Guerilla confirmed the news before the weekend on Twitter when it shared an open call for at least three dozen developers to join the studio's "Single-Player (SP), Online Projects (OP), and External Project (EP) teams."
Join Guerrilla in Amsterdam as we work to expand the world of Horizon
It's an exciting time to join us! We have many open roles across multiple departments, so check them out on our Careers page and apply today!https://t.co/G9tvnSkQQQ pic.twitter.com/Xqab1JGabV
— Life at Guerrilla (@LifeAtGuerrilla) December 16, 2022
Earlier rumors suggested the existence of a multiplayer Horizon title, and Guerilla's post all but confirms this. But, while Sony Interactive Entertainment and Guerilla refuse to explain what its actual plans are for the Horizon MMO, the job listings do give us a glimpse of what to expect. For example, the narrative designer will come up with a "deep, complex story" while the game writer will have to think about "compelling ideas for quests, factions, and backstories." Finally, the hired lead combat designer will need to draw up enemies with an emphasis on "exciting combat with cooperative elements."
Both Horizon Zero Dawn and Forbidden West are narrative-driven single-player titles, so it'll be interesting to see how Guerilla will incorporate both "multiplayer" and "online" elements in the same universe.
For what it's worth, Sony and Guerilla have been open to experimentation with the Horizon franchise. The aforementioned upcoming VR title will feature an all-new protagonist and it will let players explore new areas. A Horizon MMO feels like an evolution of that.
Coincidentally, this is in line with Sony's earlier statement about its plans to release several live-service titles by 2026. The Horizon MMO is likely a part of this plan and the multiplayer-centric spin-off to The Last of Us.
It's unknown when the Horizon MMO will be out, but don't expect it to be released until the tail end of the lifecycle of the PlayStation 5.