1- Roll back race/class choices, at least cleanse it a bit. Tauren paladins makes 0 fucking sense (example)
2- Bring back unique class buffs.
3- Remove high end loot from M+, it belongs in raids only. Maybe let M+ drop badges which can only drop there and give access to some nice HC level gear vendor.
4- Remove the layers of uncalled for complexity in raid fights, stop designing around addons and make fights more simple. They can perfectly be challenging without having to rely on addons.
5- Stop the play the patch mentality. Loot from the first raid of the expansion should be able to be carried to the last. Naturally, upgrades and replacements will happen, but why would some very specific items be able to carry on?
6- Drop all the new patch new system ideas.
7- Remove or regulate ilevel.
8- Remove Mythic raiding and tune HC up. Having LFR is fine for people who want to see the fights and don´t care about loot, fair enough. Normal mode and HC are more than enough. Or else 1 level of difficulty and hard modes avaiable like in Ulduar.
In short, make retail World of Warcraft again.