You may not like some of these things... let me know how you feel about it.
DUNGEONS
Less dungeons on release with more added later themed for introduction to the raid content. (Like ICC had) This would spice up Mythic plus every time a new patch was released on top of the affixes.
Removal of Mythic plus gear in favor of a emblem system. Receiving emblems based on the top Mythic Key you did the previous week usable at vendors for raid level misc gear that isn't part of the tier set (cloaks, rings, braces, etc.)
RAID
I'd like to see reduction/uniformity in raid sizes.
A flat 10m across the board would not only reduce barriers to entry on Mythic but would also breed more competition for the world first races. It would take less time to adjust and tune the raid tiers and in the current state of the game finding a dedicated group of 20players willing to do mythic on some servers is very difficult. Uniformity in tuning would also open up the opportunity for Mythic Plus or challenge mode Raids.(HOWEVER, I AM ONLY IN FAVOR OF THIS IF THERE IS NO INCREASED ILVL ATTACHED TO THE GEAR) Maybe a leader board or cool vanity items (xmog, mounts, titles)
I also believe that LFR difficulty should be removed and replaced with normal removing one ilvl tier from inflating numbers. I know LFR is an evergreen feature that will not be removed but the difficulty should be bumped up and the amount of people should be reduced. This would allow groups to fill faster. Also giving incentive for tanks and healers to clear the raid more than once a week (gold, emblems for a special vendor or maybe just a chance of a random piece of gear upon second completion of the wing)
Classes
We don't need a new race for awhile we've received more races in the past 2 expansion than we have since launch. I would however love to see a similar system to allied races made for classes slowly released over the course of an expansion or two.
A new talent systems that combines the old and new style.
PVP
Please for the love of God remove the fucking upgrade system. I do not want to have to grind 150k honor to get a fully upgraded honor set again.
Vendors that sell gear for reduced honor prices as the expansion moves forward. If a weapon cost 3500 honor at the start of the xpac it should be reduced by 1k every season. Place non-elite set on vendor upon season completion for honor as the initial cost of the base set from previous season.
If the honor gear is 100 the conq gear should be no more than 20 ilvls above that. Over the course of an expansion you'd need 5 seasons to reach the amount of ilvl inflation we currently have.
Also can we please get some new battlegrounds... or a Revamp of SoA :.(
Professions
I'd love to see a rework on professions. They're become almost meaningless. I'd like to see them bring back the buffs for harvesting professions. Also would love to see recipes for high level gear that is actually relevant again.
Profession specific gear slot required to upgrade your profession benefit. (I.e. if you have skinning you get a base 1% haste buff from your skinning knife, if you craft an uncommon blade you get a 2%, rare 3% epic 4%) it doesn't necessarily have to have those stat values just an example. It doesn't even need to increase player power. It could just increase the harvest rate or crafting rate, reduce material cost etc.)
Gear
As I've said through this entire rant item level inflation is a problem so I'd like to see that addressed before we are back at 500ilvl by the end of next xpac.
Return of colored/meta sockets.
Items outside of trinkets that have unique effects.
Anyways those are my thoughts I don't care too much about the setting of the expansion as long as it's not... to far out there.
P.S. remove artificially time gated content entirely just let us play the game.