Killzone was the game that put Guerrilla Games on the spotlight however Horizon Zero Dawn gave them critical acclaim and became their best selling project so far.
The last time Killzone was released was just at the start of this generation with Killzone Shadow Fall as the launch title for the PS4. It has been close to 5 years now, during which Guerrilla Games has released Horizon Zero Dawn in 2017 and its expansion The Frozen Wild. They are currently more than 250 people strong as per Linkedin which is s substantial number, but not for multiple projects.
After the massive success of Horizon Zero Dawn, a sequel was almost guaranteed to happen, but what about Killzone? It might seem like Guerrilla Games are working on a new Killzone based on their recent hires which include two industry veterans in the first person online multiplayer genre. They are the game director and game designer of Rainbow Six Siege: Simon Larouche and Chris Lee respectively (via resetERA).
Simon Larouche has previously worked with Guerrilla Games on Killzone 2, so his return does seem rather interesting right now. Guerrilla Games might not have the manpower right now for multiple projects but they are looking to add 150 more developers which sounds like it is potentially for 2 parallel projects in development, which could be a new Killzone and Horizon sequel.
In any case, this is an interesting rumor because it suggests that Killzone fans don’t have to wait long to find out the fate of their favorite IP because it is entirely possible that a new game is on the horizon.