I heard that the next Horizon has only just begun being created a month or so ago. I don't have faith it could make any launch window.
Frozen Wilds was released on November 7th, 2017, but the original game released in March 2017 there is no way after the success of ZD that they hadn't started pre-production in probably May 2017... while part of the team worked on Frozen Wilds. Not all of them would have worked on DLC.
Now taking that into consideration... by March 2019 that will be two years working on the game, and at least another 7 months for a launch around October 2019. Now assuming a console launch of 2019, I think its tight but with the engine already done, I dont see it as a big problem. Doing it as a duel launch with TLOU2... that would be enough of a great lineup along with Knack 3 :D
But yes 2020 would certainly allow more time to make a bigger wider changing game than 1. Personally I believe a 2020 release of the next console, and that would give a few studios enough time to get started on their next projects.