Exploration

Fundamentally, all of this burns down to the "Suramarization" of WoW. Suramar in general was insofar noteworthy as it (in my humble opinion) is the best endgame area (well, endgame until 7.2) in the game so far. Not only did it lack the bleak atmosphere of most of the other endgame areas (Burning Steppes, Silithus, Netherstorm, Shadowmoon, Icecrown, Shadow Highlands, Dread Wastes etc.) - Suramar as a questing area was simply more interesting. The list of the following features of Suramar should be implemented far more generously and, if feasible, expanded on:

• Expansion hubs: The Suramar model would be a good fit for future central hubs instead of solely relying on the sanctuary model. All the other cities have basically been reduced to their flight point, the bank, the auction house, the profession trainers and the occasional quest giver - Suramar on the other hand has a large amount of unique quests and a decent amount of repeatable quests. The downside is that it has only that, and nearly all of the conveniences of the actual cities are absent here even though traders, teachers etc. would actually be a good fit. Combine the best of both worlds - create quest hubs that combine all the necessities and conveniences of adventuring (like you'll find in sanctuaries like BC Shattrath, Dalaran etc.), while they simultaneously function as a questing area like Suramar did. 
• Endgame hubs: For endgame questing areas: taking control of a local hub and expanding/upgrading it step-by-step (the first time this happened since WotLK as far as I can tell) over a longer period while gaining access to different quests etc.
• Wealth of details: What contributes greatly to the atmosphere of Suramar is the attention to detail the development team has given to it; all the little things that lack in other cities - a marketplace with traders, an actual zoo deserving of that name, a high society area, more than one shop of each sort (there are quite a number of tailors in Suramar, it seems), opium lair-like houses, gardens, florists, I even found an actual classroom - all stuff that is mostly or completely absent from any other city.
• Hidden regions/places: Places that aren't easily accessible or aren't accessible at all without having achieved certain skills or perks. Skettis in TBC, which was only available for characters who had learned to fly, would be a good example of this - places you've only heard from over the entire expansion, but were only able to find after you have been enabled to "unlock" this sort of content.
• Hidden elements in general: for starters, most elements in the optional mechanics-section below wasn't something in plain sight, but had to be actively searched for; but also things like mostly overlooked vendors who sell more than food or generic crafting materials, but unique things instead (like the one in the zoo who sold a unique toy) - or use this opportunity to implement somewhat-hidden quests, like the obscure and entirely optional sprite darter quest back in WoW classic; or unique schematics and immersion rewards that can be used elsewhere.
•   Optional mechanics: A considerable amount of them, making an area more interesting to play in:
  • Adding a unique currency (in this case: ancient mana) as an additional element with everything around it (searching upgrades for one's currency pool, always on the hunt for more of said currency, gated content that has to be unlocked using that currency etc.). Also, the fact that gathering that currency wasn't through grinding mobs, but through finding "treasure patches" with lots of them in it made this, coupled with the hard ceiling, probably better done than the general grind.
  • Hidden elements or mini-quests for gaining access to zone-wide buffs (leyline feeds)
  • Hidden elements or mini-quests for gaining more mobility (telemancy portals)
  •  Elements of stealth, obfuscation or disguise (Shal'dorei glamour in the confines of the city)
  •  An army-based minigame (Training the Withered). A nice idea, but it should be more balanced and yield better rewards - and most importantly: remain interesting even after one has achieved exalted with the Nightfallen.