As big as a major story implication to as minor as a class mechanic adjustment
Mine: Dynamic seasons in at least one zone. For example arathi highlands shifting from grass to snow, nothing crazy like seeing the pixels fall from the sky, but like the tree mount that shifts every 3 months.
High Elves as a neutral race with all elven hairstyles plus some new ones and every hair color supported by the game. And they can be every class*
I want to see Rommath in a starring role. With a new unique model.
I want some actual conflict. An actual story that doesn’t involve forced contrivance and coincidence.
We’ve only gotten the vuagest suggestion of what part of Midnight may pertain to. Until like two weeks ago we didn’t know we were going too Karesh like immediately.
That being said, I want The Horde and the Alliance to quit the light over-the clothes petting and go back to headbutted each other over nonsense
I want them to not abandon Delves after TWW like they do with every other expansion feature. I want to adventure in more locations!
I want our Delve companion to not be someone that i find a constant source of frustration because they have such a large hitbox for some inexplicable reason.
Also, some new collecting achievements would be nice, get out ahead of us like you did with the pet one, no harm in an achievement existing and most people not getting to it right away, give us a goal to strive for!
If they add a void invasion event like the primalist invasions in DF I hope it works a bit like the curse of darkness in BG3 where you take damage out of specific safe zones and have to take account of that while doing the event.
Also Suramar and Darkshore updates in addition to what I hope will be an extensive update of Quel’thalas.
I could be the only one desiring to fight trolls because I hope we fight the Amani Tribe, they have such a great motivation to target the blood elves and horde since they lost their lands and Zul’jin to which they began performing darker rituals that affected the Loa. Maybe even get a later patch where we go to Zul’Dare to prevent them from unearthing something that can make them unstoppable. 
edit: Like I said, i’m the only one so it should be advised to avoid villain batting the Amani Tribe. Most I hear is the community wanting to fight Dar’khan, the Arathi Emperor, and Xal’atath. 
Could do like in WC3 where “elves” were basically visually identical except for skin color (the high and blood elf archer and night elf sentinel are the same unit, as are Shandris and Sylvanas in ROC) and put all the cosmetics on one race at this rate
Ogres, playable.
All races opened to all classes, but Demon Hunter and Evoker likely wouldn’t happen.
Transmog restrictions lifted, save for Tier sets.
Nothing even resembling Cyrce’s, the delve belt, or Reshii Wraps, although it’s sadly likely there will be one per patch from here on out (I think they’ll bring back Artifact weapons in The Last Titan, something like “Titanforged” weapons).
Actual polish applied across the UI (e.g. still no combined bank, still using old art in places, old icon borders, etc) along with more built-in customization (e.g., color options for cooldown numbers or scaling for nameplates).
Related, a revamp of Collections to include metadata on EVERY item and general cleanup for all existing elements.
Oh yeah: remove all faction locks in collections.
more customizations and glyphs.
I’d like the to see the OG developers back in charge.
That would be cool, but I don’t think they’d want to take worse conditions and pay than whatever they have now.
Nah, Blizzard is a dying company and not the jewel in the career crown it once was regarded as.
Still, you can dream…
I want to see love bloom on the battlefield <3
1.) Lorethemar to come out of it alive, still relevant, and in power.
2.) I know its a long shot, but a new class like tinker or something.
3.) A new carpentry profession for player housing.
4.) New hero talent trees for every class.
I’ve always wanted this like the tilesets they used in Warcraft II and III.