Given how fun MoP: Remix has mostly been, I would like to see them apply the Remix model to Original WoW.
Think about it. Playing with modern character models, classes, races, talent trees, and best of all Flying mounts.
Some QoL improvements:
Quest items and mounts no longer taking up bag space.
Bags larger than 16 slots.
Account wide mounts and keys.
Armor and weapons having useful stats on them.
Transmog.
LFG/LFR system.
Transmog ideas: updated textures on all the armor and weapons sets.
Multiple Keyring belts based off how many dungeon keys you’ve gotten.
Might be the closest thing we’ll ever get to the version of WoW the devs wanted to make in 2004 but where limited by the technology and inexperience (in terms of making an MMO) of the time.
Might be something fun for an anniversary event.