Level 60 completionism
Oh I'm probably going to get roasted for this. I really hate the "every zone is your level" mechanic in retail... but what about just one toggle?
There's a lot of great content and you have to pick and choose which areas you get to experience on your leveling journey... which is great! A different story every time. But maybe you're not an altoholic like me, or maybe your both an altoholic and a completionist.
What if, at level 60, you can switch back and forth between Azeroth as it is, and the exact same Azeroth but with all of the mobs and stuff adjusted for level 60? Not even updated rewards - well, maybe more coin for the time. Just a way to go back and do all the quests in the zones you didn't experience in this playthrough and have it feel like a challenge instead of just walking through all the low-level quests. Hit a portal in any capital city and bam, all the mobs are your level. Go back to the and portal any time to rejoin "regular" players and help lower level friends.
Experience the entire breadth of the world without being that one weird level 60 one-shotting everything in Red Ridge for completionism.