Getting rid of the DM would change the game the most--someone has to prepare and run it. The others have all been dispensed with in various other RPGs; I think there are a few indie RPGs with no game master (I think InSpectres?) but it's not a popular choice.
I think someday there will be strong enough support for playing without a GM that people will argue that they've got rid of the DM. There are a lot of ways to do, or
not do, the prep, and just as many ways to determine results of actions.
Zero to Hero. Which is a little different than just levels. Lots of games without levels improve PCs as they go.
LAter you change to "progression", but I don't know how that can possibly be meaningfully different from levels. I can certainly see a dnd in the future that either starts out as heroes and never quite gets to demi-god tier, or a game where you never
really graduate from "zero" to "hero".
My own pick is Class. I really do think dnd requires classes to maintain any sort of identity. Not only that, there will always be plenty of non dnd classless games. DnD
is the fighter, wizard, rogue, cleric, paladin, druid, bard, ranger, monk, warlock, etc. You can remove or add to the list, but if there aren't archetypical class packages that imply a certain approach and style for a specific character, it ain't dnd.