Let's just jump right into it. What stuff do I think would be awesome?
An overhaul of the leveling/alt process
I definitely think we need to reconsider how we deal with alts. I'd love to see a system that reduced the time you had to spend getting them leveled once you got the first one to max - heirlooms work for that, but that long-rumored heirloom tab would make it a lot easier. Another thing which would definitely help are more heirlooms like Hellscream's Decapitator - our current heirlooms don't take us all the way to max level the way it will, and it's also useful now as a raid-level item an alt can make use of. It's an experiment I hope they continue.
I've seen some arguments that we should be able to start a high-level alt to skip over a lot of the leveling process, especially as we get closer and closer to the likely new level cap of 100. If that was implemented, it would need to be done carefully, but I'd definitely support some way to preview a class at or near max level so you could figure out if you'll like it or cut down on having to do the full 1 to 100 (or whatever it ends up being) march again.
Let every race be every class
I used to be totally opposed to this - I'd argue up and down that it doesn't fit the lore for hooved races to be rogues, or for pandaren to be death knights. But lately I've come to the conclusion that I just don't care about that compared to the idea that someone might play and enjoy a combination. Sure, I wouldn't play a gnome hunter. But someone else would, and they'd like doing so, and if that's the case then I think it should be in the game and we can either invent new lore to justify it, or just ignore it as an edge case. You're the one pandaren who has the really bad fortune of going exploring from the Wandering Isle in the Eastern Plaguelands and Arthas got you, and his undead whimsy just couldn't resist making you a death knight. You found the secrets of high elf druids and now you're a rare blood elf using druid magic. You're an impossibly stealthy tauren.
I admit it - despite fears of ability bloat, I love new talents. Love love love them. I want to see them. Some abilities like Bladestorm, Avatar, Gorefiend's Grasp, Living Bomb are just cool. One of the things I really like about the talent system that debuted in Mists of Pandaria is that it made each character feel more individual based on which one of the three talents per tier you picked. I would definitely like to see that continued.
Alternate races and race model options
I expect we'll get new character models, and this is related to that, but not quite the same thing. I'd really like to see alternate models for playable races. Not every race has obvious examples for this, but some come to mind immediately - draenei should have playable broken, tauren have both taunka and yaungol which could be incorporated (as well as Grimtotem face tattoos), dwarves have Wildhammers and Dark Irons, orcs could be brown-skinned mag'har or red-skinned chaos orcs, blood elves could get a high elf model (or Alliance could get high elves as a race change option).
Another possibility and something I've wanted to see for years would be to allow for faction change without race change. You couldn't roll an Alliance tauren, for instance, but if you switched factions you could retain your race, potentially with small differences (like Alliance high elves or Grimtotem, perhaps draenei who went Horde could play as broken from the Swamp of Sorrows).
Even if we don't get that (and I don't expect we will) I'd really like a few sliders on my character creation options. It's been a long time since 2004 - most MMO's have embraced the ability to be a tall, short, thin, or fat character. A little customization would be nice.
Diablo III's Transmog system
Yes, I know I'm hammering this one into the ground. I really really want this one. Tell you what, I won't count that and you'll get a brand new fifth feature, but I'm never letting this one go.
Two new races
You could argue that we don't need new races, and I'd absolutely agree. We don't. That doesn't change the fact that I want them. There's lots of possibilities out there - vrykul, ogres, furbolg, tol'vir, mantid (come on we all want mantid), saurok, hozen, ethereals, naga to name just a few. New races are fun. New races mean new warriors for me to level. It's win-win.
I'd like it if we went with some non-obvious matches, so not ogres for Horde (they'd be too much like tauren, a gigantic race towering over everyone else) but that's just a personal thing, I get why most people would expect ogres to end up Horde side, it's tradition. Mok'Nathal would be a good compromise there. Ultimately, though, I've enjoyed pretty much every new race we've gotten, and I'd be interested in seeing what we got.
And those are five things I want to see. Will we get any of them? I have no idea. But we'll find out Friday.