I would really like to see Capital Cities receiving some love. All of them. Some more than others. Some I think should be replaced. Some should be reclaimed. Some should be revamped and some should be expanded upon.
I think what would be great is if we get a racial questline for every race, something that shows us doing something with our capital. Like Night Elves growing another tree, over time. Blood Elves rebuilding the second half of Silvermoon. Gilneans and Gnomes getting their capitals back. Then at the end of that questline, we'd be able to pick our own little house in an instanced version of some part of the city.
Some ideas
Humans - Stormwind (revamp, generally big enough, but graphical update needed)
Dwarves - Ironforge (revamp, maybe adding some underground portion to it)
Night Elves - Something'drassil (new tree, looking like an actual tree, bigger than before, with new starting area for Night Elves)
Gnomes - Gnomeregan (actually reclaiming it this time, maybe not as big as the dungeon was)
Draenei - Exodar (maybe flying in the sky? I don't have much here)
Worgen - Gilneas (reclaiming it, we've never actually seen it like a real city, which is a real shame)
Orcs - Orgrimmar (pretty much ok the way it is, maybe getting the Goblins out of there, cleaning up their mess, using the underground portion from SoO)
Trolls - Echo Isles (putting some more stuff on those islands)
Tauren - Thunder Bluff (revamp I guess, don't have many ideas here)
Blood Elves - Silvermoon (rebuild the second half)
Forsaken - Tirisfal (clean up the zone, abandon the sewers and put your capital in the middle of the zone and use some of the Northrend Forsaken architecture that they also used when they revamped Tirisfal with Cataclysm)
Goblins - Kezan, Undermine (fuck it, just give them the whole thing)
Pandaren - no idea, go back to Pandaria
Some of the Allied Races have obvious choices, like Suramar, Thunder Totem or Shadowforge City. Some are a bit harder, and maybe require something new to come up with. Maybe not every Allied Race needs its own city. Mag'har are with the other orcs in Orgrimmar for example.
Generally the idea is, all of them could need a graphical overhaul, and I would like to see all of them get a bit bigger, like Suramar, Boralus and Dazar'alor.