If Riot had the same release schedule that they do now since Season 1.
Give or take a few releases,
would be the next champion to come out and
would be released in Late spring / Early Summer of 2021.
What I did was have 6-7 champions a year be released, or a champion released every 45 - 60 days with slight influxes in favor of quicker releases. Very rough estimates but pretty accurate results despite the sloppy method in my opinion.