Honestly, WoW imo was best in Vanilla mostly because it debuted with like 50~ zones ripe for exploration. I remember when I started playing I just wanted to explore rather than quest or level. So I just went around and grabbed all the flight paths at like level 15 or some shit. That was fun as hell, even though I died a bunch.
I feel like the problem with these new expansions is there's just not enough new content in them. They normally come out, have like 5-7 new zones with what, like 50 to 100 quests per zone, and that's it. Nothing will compare to Vanilla and having the ability to choose what zone to play next, because there were like 4 or 5 zones for each level area, so if you didn't like this place, just go to that place. I for one hated Thousand Needles, as I've always disliked multi elevation zones, so I skipped that and went somewhere else. You don't really get that ability anymore, Northrend had a bunch of different zones but I don't think any of the expansions after that offered enough variety. Maybe MOP, but I feel like two or three of those zones were pretty barren overall, especially on a play through last time I subbed when I reran the entire xpac.
Blizz just needs to hire more people to create more content for the game. Finish the unfinished zones next time. Cataclysm abandoned that underwater zone real quick, then Warlords of Draenor was like 25% completed from what they wanted to do, Legion was pretty packed full of content, but then BFA was a giant ass grind fest and then Shadowlands disappointed everyone with even more grind shit.
Simply hire more people to create more content. I remember in Dragonflight they'd tout this brand new big patch with new quests after like 2 months of nothing, and it was a questline you could complete in like maybe 30 minutes. All the hype for nothing.