to the question what would make ME play retail again:
Removal of one raid difficulty, merging normal and heroic into one, with the first boss being normal first boss, and the final boss being heroic final boss - with all in-between scaling up.
Merge M0 and Heroic dungeons
Gearing to be closer to older expansions, so long as the intention is that the best pvp gear comes from pvp and the best pve gear comes from pve.
Removal of all the additional systems, I just want my character to naturally grow and progress throughout the expansion from gear and content.
Unleash the creativity of the visual team, let them go "crazy" with their gear designs - bring back tier sets, and let them move away from the rigid adherence to armor type. I am happy for a priest set to look closer to a paladin, a pally set to look a bit cloth, a rogue set to look closer to mail, a hunter set to just be animal hides rather than mail etc etc etc - i would prefer to see cool, fun, creative designs that fit the class closely rather than something constrained by armor type.
I realize some of these are "looking backwards" rather than forwards, but its a combination - I have no problem with them reusing things that worked over and over again, while still introducing new things, like M+ for example.
And then, on to the biggest one for me, and the one likely to get the angriest response, if there is one at all: Its time to completely rework the classes, starting from the ground up. This is my thoughts on the subject - their pursuit of perfect class balance has taken ALL the rough edges away from the classes, and thats a real problem for me, because those rough edges were the "oh shit! fuck yeah!" moments. Those moments where the warrior got a boner because there was a pack of 12 mobs spawning and all his cds were up and bladestorm was ready to go - the moment were the pally could AOE taunt and be a hero - the moment where the group almost died, but the resto druid was able to pull everyone back to max hp with one well buffed and timed tranq. The times were there was a target for my assassination rogue to tunnel a boss for the entire encounter, because his sustained ST was so godamn strong, that we had him ignore all adds and just tunnel the boss. I personally feel like everyone has to be "good" at everything now, and that, to me at least, means no one is good at anything.
To be clear, I think M+ is a decent system, however it has resulted in Blizzard painting themselves into a corner, which is an amusing place to find themselves since they did EXACTLY the same thing with Arena - made it so all classes needed to be pretty similar, otherwise no one would bring X or Y specs. Using M+, imagine bringing a pure ST spec now - why on earth would you?
tldr return to previously successful and popular systems, while still introducing new ways to play and new gamemodes, just without all the throwaway nonsense. Trim the fat with difficulties, and streamline the entire game to be more straightforward for newer players, while still giving those of us who want to do challenging content an outlet for that.
Lastly, this is what I personally want, im not saying these things would be best for the game, just best for me personally.