Just a few things off the top of my head. One thing i always hoped for is World Quests for every zone that scale to your current level. Have a like 50 different ones for each zone that rotate weekly. Rewards can be everything, from battlepet tokens, to gold, to rep tokens. Also let the rep for each faction continue for those max rep chests. Make like 5 of them active in a zone and rotating.
Also some stuff like invasions in Legion but more common, like roving warbands or something in a specific zone, that have some specific quests.
And as long as I’m going, might as well make all quests scale with level, so it will be fun to redo old stuff and have it be a minor challenge at least.
Also i wouldn’t mind if they expanded on the battlepet gameplay. An overhaul to how pet battles work maybe (like you become you’re battlepet and fight the other in a more real time combat). Add some more Battlepet dungeons.
As mentioned earlier, an update to Archeology, make the profession smoother and add more lore. Can even upgrade older sites and factions to contain lore concerning newer expansions and the like.
Adding more puzzles and treasures throughout all expansions. So if you’re just flying and exploring you might stumble upon random new little tidbits you can engage with. Make them also random appear and disappear.
In short make everything more dynamic. World of Warcraft has a massive world, that’s for the major part been left behind as soon as a new expansion arrives. That shouldn’t be the case in my opinion.