Honestly, after reading through some of the replies, I realized most people already mentioned a lot of the bigger/more important story threads I’d want to see, so I think I’m going to lean a little harder into the niche lore-nerd / gameplay-addons side of things because… well, that’s my specialty. lol 
TL;DR - Story and lore excuse to add a new race, class, or a ton of customization to the game. I'm a very niche player, and I know what tends to drag me and the people I play with back into games. Can't speak for anyone else, but this is the plain—unadulterated truth of my bias requests. lol
First, I really want to explore the Iridikron side of the story. Do we actually know what happened to the Dragon companion of “the Goddess” from that old Harranir story? Is Ula’tek related to it somehow, or was that just a nightmare interpretation of Y’Shaarj before he was ripped out and created the Rift of Aln? And is there any connection with Hakkar since they share some very similar aesthetics? I dunno, there are too many weird little pieces there for my brain to leave alone. lol
And while we’re poking at unresolved dragon stuff… can we PLEASE do something with Chromatus already? 
That is such an obvious dangling plot point. Maybe even use him as an excuse to finally bring up that Legion legendary weapon, the Glaive of the Aspects. The thing was supposed to be on the level of the Dragon Soul, and since Chromatus is basically impossible to destroy, which is why he was sealed away, we could use it against him. If we’re going to fight something like Chromatus, that seems like a pretty good excuse to look that up again.
Also, I’m obviously not going to complain if that somehow ends with me getting a new giant Draconic Glaive for my DH. Just saying lol! 
And if we’re already doing dragon story stuff, PLEASE give Dracthyr/Evokers more customization while we’re at it. More Hero Spec possibilities, weird/dangerous combinations of Aspect powers we haven’t seen before, and just… more stuff in the character creator. I feel like there’s still a ton of untapped potential there.
Then I started thinking about the whole “Dreadlords infiltrating Life” thing and my brain went completely off the rails. lol
Because… what if we could actually play a Dreadlord?
Not saying this should ever happen because I realize the MMO space would probably not be the best place for it, but I love the idea of a Nathrezim who actually chose to turn against the others, like Lothraxion did. Their whole identity is infiltration and stealing/imitating the powers of other beings, so I’d love to see a playable version lean into that. Almost like an inverted reflection of the Champions of Azeroth: take iconic abilities associated with the existing classes and twist them through Nathrezim death/vampiric magic. 
Basically a fake cross-class that fights like a Dreadlord instead of just being “another class.” Give me some completely cursed combination of iconic WoW abilities and let me make the most suspicious character imaginable! 
…yeah, this is probably a terrible idea for an MMO. But as a crazy single-player story/side concept? I’d absolutely eat it up! Especially if we got to see Sire Denathrius again. Anything to see that magnificent bastard return. lol 
I’m also still very curious what Azshara and the Naga have actually been up to. If that eventually leads into the suppose Fifth Old God stuff, even better. And obviously I would not complain if somehow that became the excuse to finally make Naga playable.
The Arathi Empire is another one I’d like to see followed up on, although I’ll admit I’m still more curious about the Naga/Azshara stuff. I’m just not sure how much longer Blizzard wants to leave that thread sitting there.
And then there’s Illidan.
If he ever comes back, I would honestly love to see one final nostalgic team-up in Northrend with his old WC3 crew, Vashj and Kael’thas. Just once. Just to see if I have any nostalgia left in me after TBC killed most of it. 
I don’t even need that for myself at this point. It’s more for my friends and the kids who grew up loving those characters way more than I ever did. Sometimes you gotta let the next generation have their “OH MY GOD THEY’RE ALL TOGETHER AGAIN” moment. lol
So I guess my answers are a little selfish (ok, FINE super selfish! haha
). I don’t necessarily want Blizzard to make another expansion around exactly what I think is cool. I’m just the kind of player who gets pulled back in when the story gives me an excuse to make some ridiculous cool character, unlock some weird new customization, get a crazy weapon, or finally do something with a lore thread that’s been sitting there forever.
Anyway, if you reach this far into the read without going mad or crazy, congratulation your full proof against the void whispers that linger in my mind!
