Honestly I think the best route would be to squish all the 60s back down to 50, and reset the iLevels of the gear to what their BFA equivalents would be.
Cata max level gets put to 30, Mop to 35, etc, except leave BFA the main leveling route for beginners. They just go to the Dragon Isles instead of the Shadowlands when they reach the appropriate level.
This way Blizzard doesn’t have to remake numbers to an ever-higher level just to squish again in a couple of Expansions, as it would be the expected standard.
The alternative would be side-ways progression, in that the storyline and alternative Talent-style progression is made available. A few MMORPGs have done that through the years, Dark Age of Camelot, for example. While there was PvP skills to level up, other expansions provided new tools to play the game with here and there, but they weren’t more powerful than what you could get from a drop, just power was improved in a different way.