Preface. I am someone who doesn't really care about the quality of leveling, I can't imagine that changes even with the best intention would be able to blow me away. But I am fine with this! I enjoy leveling when I can, and I'm not too grumpy when I am not enjoying it. Up until Legion and BfA. Boy, I didn't have a good time. It was a torture. I hate it. (I have many many alts at level 90, and I just can't get myself leveling them in the last 2 expansions.)
I love leveling alts with op enchants and heirlooms and xp potions and WoD leveling was especially great for this. My main is different. It is usually a slog, but often it is a somewhat enjoyable slog (BC-WoD)
I would really like hubs back. Legion and BfA both gives you 4-6 quests at time, but anyone remembers Wintergarde Keep for example? Where you immediately upon arrival, got 10-12-ish quests, most of them situated at one place, or at least you could plan ahead where to go. Not anymore in Legion and BfA. Your hand is held tight, and papa questdeveloper drags you through the whole zone only with a couple of quests at a time. You go exactly where papadev tells you, in the exact order as he tells you. No leeway, no flexibility, no sightseeing, no nothing. This you can see everywhere in WoW, why in leveling tho? I have no idea.
I would really like if you could open up bonus objectives like in WoD that would give huge (I say it again: HUGE) amount of xp. Maybe not so easily like in wod.
I would really like GW2 progressive event type of things in wow.
More zones than it's needed to cap like in BC and Wrath, so you can choose your path, giving you fresh start with alts.
Maybe a couple of areas that would change permanently after you complete it. Progressive zones with patches.
They are experimenting with it in BfA, hopefully they are expanding on them: quests with choice.
Zones that are not necessarily fully packed with mobs. Like Valley of the four winds. I loved mining/herb farming there, there were not many enemies, beautiful scenery.
Completing a zone (quests, rares, "vistas") should give rewards immediately (flying), even if you are under level cap.
Leveling once was like (felt like) swimming with the flow. In Legion and BfA it feels more like swimming against it.