Forza in Early 2022 would be fine since Forza 7 and Horizon 4 are already on Game Pass. Those will be two racing games to play for those buying a Series X|S.
My thinking is Forza 8 in either late 2021 or early 2022 and then Horizon 5 in 2023. I think that comes out after Fable, which I'm putting down for late 2022.
The Forza and Fable team are different teams. If FM8 is 2021 then FH5 will br 2022. And if FM8 drops in 2022 then FH5 should be 2023.
If wete not getting a new Forza til 2022 I hope we get some sorta big next gen improvements for Series X and S next year for Horizom 4.