Horizon multiplayer game is reportedly in the works at Guerrilla Games. It's possible that this would either be a standalone online spin-off, or part of Horizon 3.
According to reports, Sony had been keen to include co-op in the upcoming sequel Horizon Forbidden West but it was ultimately decided that such feature would be saved for a future game, which could be a standalone spin-off title or Horizon 3.
Since it's been confirmed that Horizon Forbidden West won't have multiplayer nor co-op, it's safe to say that a standalone multiplayer Horizon title is now a real possibility, unless, Sony's plan is to get Guerrilla to work on the third game in the franchise right away.
One of the job listings on Guerilla's official website includes a senior social systems designer position, who would be responsible for exploring "creative ways players can connect with others in-game" and who has "a passion for player communities and social features."
There's also a leaked artwork from 2014, showing a co-op feature in Horizon Zero Dawn, which confirms that Sony and Guerrilla had plans for a multiplayer game for quite a while.
As expected, the job listing does not mention that this is actually a Horizon franchise that will be getting a multiplayer component but VGC report reveal that there are plans for such a title.
Guess those Killzone is making a comeback rumours were pre-mature after all. It doesn't seem like there are any plans for this shooter to return anytime soon.