Wouldn´t it be nice if...
You get to level a character in any classic to level cap, then you can transfer that character to retail to do the actual end game.
To make it work flawless and fairly, there could be a few basic rules. Mounts, gear or no longer obtainable items are a big factor, so ignore them for a second (that would ofc be sorted somehow).
It hurts none but it adds an extra play style option to the game
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The point is:
Leveling back then was so much better for many of us. You did get to become one with your character, it was a very personal and immersive experience and you got the chance to explore every detail of Azeroth in much more depth than todays experience. Doing dungeons felt like actually doing dungeons, rather than rushing them for quick xp.
There could be realms dedicated for this, maybe 1 is enough. So people on those realms are exclusively there to level up... easier life finding groups and help.
I know that many people feel the same way, but the problem is end game. We did it so much back then.. I don´t see the point doing it again. Leveling is different as it takes far longer and is far more diverse as well as you progress a character.