Not making a new killzone was the best idea ever. the team would have been sick of making the same game over and over.In theory, Guerrilla rotating between Killzone and Horizon would be a good idea. However, it only works effectively if both IPs are really good, like Uncharted and The Last of Us from Naughty Dog. Killzone is not at that level.
This has given them a new lease of life. Maybe develope another killzone now and possibly make it semi open world? And their next game after can be Horizon 2 to launch on the ps5. That would be an awesome exclusive to push the ps5 and it would have mega hype after the reception Horizon is getting!
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