For my list, in order of importance.
1 - LFR removed for good.
2 - More server "merges" - while I'm on a high pop server it isn't fair that others are in crap. I was on one before xfering to ZJ.
3 - Separate 10 and 25 man raiding - Karazhan to Gruuls/Mags and ZA to SSC/TK again. Flex could be for both to allow for content to be viewed.
4 - Bring back class uniqueness - So what if shaman are the only ones with bloodlust? Or warlocks the only ones with curse of elements. With the separation of 10 and 25s, it would allow for content to be made again around not having all the buffs in 10man while 25s could be designed again around having each class.
5 - More unique fight mechanics - Mage/warlock tanking on bosses, mind control tanking or pet tanking. Let people use those random abilities that never get used.
6 - Redo talent trees - Make them not lame and boring like current ones. Make there be some aspect of choice in them rather than this is for this fight etc etc. These "new" ones were supposed to encourage diversity and all they did was bring everyone to the same spec quicker due to ease of change.
7 - Attunement/gated raiding - Seeing/killing the end boss without an idea of how or why you are killing him is not good for any game, mmorpg or not.
8 - Lore of people we actually care about - As much fun as random panda #5 is as a boss, it'd be a lot nicer if the things we are fighting or questing with actual have a reasonable connection to the players. I personally didn't give two craps about any single panda at any point in this whole xpac. The only lore figures I actually care for are the Klaxxi.
9 - Class based quests/items/legendaries - This cloak thing was just trash, the whole quest line was just plain lame. The warlock green fire quest on the other hand was supposedly really amazing and actually pretty hard. Put those quest people on the new xpac legendary line and we might have a good one.
More unreasonable ones....
10 - Removal of heroic raiding - A return to TBC style of zones. One difficulty for all types of people(minus flex which would be a second difficulty but don't ruin my saying!). The instances would be pretty much like the tier of TBC where there were easier bosses at the start and "heroic" bosses towards the end. No more of this clearing the same place twice thing. When I kill Illidan, I don't want to have to just kill a harder version of him next week for the "real" fight.
11 - Profession rework - Make professions realm wide on your account. If your "main" is a blacksmith/jc, all your chars on the server are the same. Move gathering to secondary so everyone can gather anything they need for said professions. Make them unique again. Currently eng is the only unique prof as it has all the "fun" stuff. No reason that can't be the same for all profs. Give alch people unique potions for buffs or random fun illusion crap where you can turn in to boss models or something. Make leatherworking drums a thing again. Give BS people weapons/armor unique to them, same for tailoring. Making having specific profs mean something outside of the stat bonuses you get.
REALLY FAR OUT THERE ONE...
12 - A system similar to Everquest AA points or Rift Planar attunement - I want there to be something to do to better my character when I log on. I don't care how small of bonus it is. I want something to do when I play outside of raiding that actually is important for my goals, ie raiding. It shouldn't just be for raiding, there should be all types of bonuses you can get.
13 - Veteran rewards of some kind - I've been playing this game since it came out, you(Blizzard) have made TONS of money off of me... give me a little perk to say thanks, is that too hard? Give me a random unique mount every 3 years or a free server xfer every year or a free level 80 for 5 years or 4 hours of 100% for 3 months, any number of things would be just sweet.
That's all I got. I don't overly care what the actual xpac is about as long as it starts moving some things in the right directions.