I could see a permanent RetroSquad replacement possibly coming in 2023; I think a year might well be a little soon for Merlin to plan a permanent replacement, whereas 2023 might let them take the results of their RetroSquad experiment in 2021 and properly implement some results.
I could seriously see some kind of big flat ride package for 2023, from one of the big manufacturers (Zamperla, Huss or Intamin, perhaps?). Although the park has supposedly purchased FFF, or a similar ride, on a permanent basis for 2022, so maybe they might do that with the other 3?