This is a good topic! Jumping onto the bandwagon:
1) As many already mentioned above, I want more character customization options. WoW is seriously lacking in that department as of right now. More of... well, everything, will make it better. In particular, I hope they add a possibility of tattoos - through WoWHead modelviewer, it is seen that Vrykul use tattooed "
shirts", which I think is a nice way out if they don't want to add it to the character creation screen as an option. Additionally, I think it is past the time for the sub-races.
P.S. Update goblin and worgen models.
2) Please, no big "KINGDOMS SHALL BUUUURRRN!" threat this time. The last two threats like that have not been kind to the lore of both factions. I seriously hope they lower the degree of world-saving for a bit and concentrate on building up factions after the loss of Varian and Vol'jin and shaping the world that's recovering from the biggest invasion ever.
Inner conflicts, tensions in the factions' hierarchy, more lore to existing playable races (it's about time we see gnomes take back Gnomeregan and trolls choose a new leader and so on, show some Pandas and dwarves around!). Faction leaders making frequent appearances as well as some other important characters such as, for example, Jaina. All in all, an attempt of factions to return to relatively peaceful coexistence with some local skirmishes and head butting here and there, with the mood growing darker as the expansion goes on, ultimately leading to N'Zoth rearing up his ugly head in the next one.
In the same key, I hope the new zones will not be level-scaling. Having a zone be level-scaled means it exists in and off itself, and should fit in the timeline at any point - as the first and the last at the same time. I think that did not work out real well, because that chopped off existance also meant nothing ever coming out of the said zones and them existing in stasis through the leveling process. For comparison, once you're done in, say, Azsuna, you return to Dalaran and start a new zone without the connection between the two. The only connects there are is a bit of Nightmare between Suramar and Val'sharah and Queen's Reprisal in Azsuna - and yet, due to the zone being in vacuum, we don't know whether Stormheim or Azsuna "canonically" happens first. On the other side, for example, the whole leveling process of WoD leading to Nagrand or WotLK where you could go through the connected storylines, in the end leading to ICC (Howling Fjord - Storm Peaks / Borean Tundra - Sholazar)
2.5) Shadowlands!
3) I also hope for old zones to be updated - not only in terms of visualization, but also with content. A buddy of mine, completely new to WoW, started playing a Panda. The amount of explanation I had to do for to him to bring him up to speed... "Right now, you're in Pandaria, although both these guys were Warchieves and died since then. Oh, he makes an appearance in the zone? Well, this is back from Cataclysm, before Pandas even appeared and before his death. This world is TBC, so neither of what I said counts. Oh, and this is Lich King. And both are before Cataclysm and Panda"... and then Pandas act like he's one of those races they'd never seen before and so on.
At the very least, I hope for updated starting locations / quest chains and visual overhaul.
4) I hope for a much lower degree of RNG. Some RNG is totally fine, it can even be, dare I say it, fun. But what we currently have? Ain't fun at all. At the very least, return of reforging to deal with extremely bad RNG to make it not so bad. Return of the actual meaningful professions and reputations: recipes, mounts, pets, some not-too-bad gear, perhaps unique transmog.
5) I want WoD PvP system back - from gearing up and way gear (both PvP and PvE) behaved in PvP to removing the templates. The idea was good, but the way it turned out is lacking. PvP is plain disastrous right now. I also wish they'd add more PvP rewards for "casual" players, that you earn based on participation instead of rating and that can only be achieved through instanced PvP, preferably, random battlegrounds (World Quests are nice, but they aren't making any favours to actual PvP which includes, y'know, PvP-ing). I also hope for PvP sets with set bonuses, and vendors where I could choose my stats.
Oh, additionally: I hope they sack Versatility and deal with the rest of the secondary stats. A +15 ilvl item should be an upgrade. But alas it is not, because it's all Vers.
6) More cosmetic stuff altogether - transmogs that are faction based (like the Wardens) and marked as Cosmetic, glyphs (for example, something like Elune's Light - turns Arcane Shots light blue instead of whatever it is, permanent armored form for doodoos, Shadow Shield to turn Power Word: Shield shadowy for SP, etc.)
7) Like the Pandaria scenarios are available through the Seat of Knowledge, I hope they make old dungeons / old world if they do revamp it / new scenarios showing off the most important fragments of Azeroth's past available through the Caverns of Time.
8) Catch up on some loose plot points - Sylvanas & Helya, Sylvanas & Greymane & Eiyr, Wrathion & Sabellian & Ebonhorn, Dragonflights altogether after the loss of Ysera (I worry for Alextrasza), Calia Menethil, Alonsus Faol, Odyn, Medivh, Jaina (+ Kalecgos)... The list can go on and on.
9) Windrunner sisters reunion. A legit one this time. From the Horde point of view, I hope that before meeting Sylvanas, Alleria visits Silvermoon and looks around / talks to the citizens and leadership, because I don't trust in the way Vereesa might have portrayed both the faction and her sister.
10) For the love of God I hope Blizzard class design team takes a step back and looks hard at how things currently are. Bring in community representatives if needed, conduct polls (even if they only reach out to a part of community, it is better than none). Many specs right now are just pitiful and those that are not still have dire problems.
Cannot say that I can't wait for another class / spec revamp and learning how to play certain class/spec combinations anew, but it is better than nothing.
11) I want a fourth spec for Hunters that is roughly what the old Survival was. Be it based on poisons, Shadow powers ala Dark Ranger or Void powers like Alleria, doesn't really matter.
While I think the best option for old Survs out there to hope for would be intergrating the spec into MM, I also think it not a good way out. Not only it brings in the problems with balancing it out, but it also washes down the "spec fantasy" itself.
12) Instead of having a row of talents be split between, say, movement and defensives (Warlocks) or single target / AoE, have a row dedicated to a single function. For example, AoE, and the ways / forms of dealing said AoE (direct damage cooldown such as Barrage or Butchery, passive effect such as Volley or Serpent Sting after Carve, a dot on CD like old Lynx Rush or Dragonsfire grenade).