I think Classic should be kept as authentic as possible. That is what it is there for - to be a faithful representation of a great game.
That said, I think you raise a valid and interesting point. Classic has flaws (some would argue that is part of its charm) and many areas that it could be improved upon. And I see you mentioned a “WoW xpac that takes goods things people like from the past and mix it with good QoL developments.”
Well… If Classic is very financially successful, maintains a strong and coherent server population, maybe that’s what you’ll get? We’ve already seen from WoD that WoW has a multiverse. So it’s not like there isn’t already an in-game reason for them to do a divergent story. Perhaps we’ll get additional future content they had once planned to add to Vanilla but cut? Dragon Isles, for instance. Or maybe a new expansion that isn’t the Burning Crusade? This is doubtful, but not beyond the realms of possibility.
As for your final question, regarding class… Well, I can’t give you any definitive answer there without knowing your tastes! I would say Warlock and Hunter are very good for leveling (pet tanking!) and also remain viable in the late game and in PvP. But bag space is often a premium for them because of Soul Shards and Ammunition.
If you like tanking? Warrior. Feral Druid and Prot Paladins are often renegated to an Off-Tank or Trash Tank role. Warriors have the best single-target threat generation in the game. It’ll be Warrior Main-Tanks who are likely to get priority on those Classic Legendaries.
Healing? I would strongly recommend Holy Priest or Holy Paladin. They are the games best healers. Resto Druid or Shaman are doable, certainly, but in terms of end-game content the first two are king.
Rogue? Savage in PvP. Probably one of the best DPS in the game with their threat-negation abilities and quickly regenerating resource (energy) allowing them to just keep on churning out their attacks without pause.
Hope that helps!