What I would like for the next expansion is a complete reset. Basically, you start from level 1 again after achieving level 120. Having atleast a character level 120 would be a prerequisite to play Shadownlands. In character creation screen you can chose to play Shadowlands or the BFA progression. But honestly… level a new character from 0 - 130 is dumb, provides nothing new and the leveling experience in retail wow is extremely boring and useless since mats/professions basically don’t count.
You start from level 1 since you die at the end of BFA, make a slower progression as it is in Classic so people can enjoy content and lore. No Flying, mounts at specific levels, completely new talent system. This is a good idea considering the huge amount of success that Classic has.
What I miss and found in Classic is that you are not some superhero, best in the world, champion of azeroth, we would be doomed without you type of character, but just another foot soldier… a small guy finding his place in the world. I would start Shadowlands as one of the soldiers fallen during the Battle of Stormwind, but not as your previous character. Considering that when you go in the Shadownlands you are basically a spirit version of yourself I doubt you have the same powers you had in your own world and as your own self. Getting used to living as a spirit in the Shadowlands means that you basically start over - new talent system, maybe new spells, new progression, new everything.
What better time to start from level 1 than after you freakin’ die :)).