Zero Dawn is done, Frozen Wilds was the only planned DLC, hence the Complete edition. Any new Horizon stuff at E3 would be a sequel. It's probably way too soon for that. More likely a 2019 reveal at the earliest, but who knows, it's not like Guerrilla is suddenly gonna go back to making Killzone again in the interim. Maybe they'll pull the ol' title card/voiceline/"now in development" teaser.
But the turnaround for anything the size of Horizon isn't gonna be just one year, or way less than that actually considering Frozen Wilds launched in November.