I started playing during Cataclysm but never actually hit the level cap and started doing end-game content until WoD, so I feel I might have an interesting perspective on this.
Personally, after trying out a Vanilla server, I like a lot of the quality of life improvements that have been brought to the game. Even though WoW is like 12 years old, it feels pretty modern, and a lot of stuff has been streamlined so that you can play the game on your terms, based around your life. For all the complaints I see about LFG and the like, WoW has evolved into a game that I feel respects the players' time, and I think it just needs a kick in the right direction to set the game back on track again. It can be done; WoW would not be the first PC game to have a renaissance years past its prime.
I'd say there are three big changes that need to happen to "revitalize" WoW and make it exciting again:
Stop trivializing leveling. At this point, leveling is super easy and is pretty much just a formality that you slog through to get to what Blizzard says is the real game, end-game content. My radical dream change would be to drop the max level BACK to 60, make leveling slower and more difficult, and give larger exp rewards for doing main story missions vs. just running LFD over and over again. With all the content and zones in the game, Blizzard could make a leveling experience that would not only be very fun, but also would give the player a ton of flexibility in where they want to level (this would work well with Legion's level scaling).
Introduce more stuff that gets players out in the world at end-game. Make major events that pop up and make players want to scramble over it. Right now, WoW's endgame is pretty much raid-or-die and I think it'd be fun to inject world PvP back in the game. Right now the only time I ever see other players around is when I'm doing my dailies. Imagine how cool it'd be if a treasure or something appeared for Horde and they had to defend it for X amount of time from Alliance players, for example.
Remove transmog. I know this is a weird one but I miss walking into Stormwind or something and seeing some raider decked on in crazy looking gear and knowing he was super hardcore. Now everyone wears gear like that and gear has just been reduced to ilvl and stats.