At one point (well before COVID-19), I remember there was one plan loosely discussed amongst WDW management where the 50th would take up a good chunk of 2021 and most or all of 2022, go straight into the TWDC's 100th anniversary in 2023 and part of 2024, and then after a brief pause, they'd transition to a celebration of America's 250th birthday in 2026, sort of similar to the bicentennial celebration they had in the 70s.
This had to be at least three years ago I think (as they were finalizing capital investments for the 50th), so I'm doubtful much of it will materialize. We already know the 50th isn't happening on the original timeline. That said, celebrations can be a good way to drive attendance and don't have to mean new attractions. You can do a special parade.