I feel like Avatar and Star Wars will take the December slots whenever one isn't appearing. Anytime an Avatar movie is not on, Star Wars will take its place. If the current slate stays this way, then 2022 will be vacant, and there will be a 3-year gap from IX. It's enough time where there will be less fatigue and a hunger for new Star Wars content. Although I doubt Disney will try to do "one a year", at least for the short term, so 2023 might go to another movie.
So basically:
2020-Avatar 2
2021-Avatar 3
2022-Star Wars (The GOT guys or you-know-who #1)
2023-Mary Poppins: Tomb of the Dragon Emperor
2024-Avatar 4
2025-Avatar 5
2026-Star Wars (GOT guys or you-know-who #2 or #3 if they want to put one of the movies in summer 2024)