While WoW may never recapture it's former glory, it seems to me that part of the appeal during its heyday was less about the game itself and more about the social world that prospered in Azeroth.
Sure, the game was great. But lots of games were great before WoW, and there are some great games out there right now. So how did WoW reach not only the pinnacle, but then remain there for nearly a decade?
I would argue that a large portion of the fanbase was more interested in visiting with their group of friends and/or their guild, and the game simply served much like any other RL hangout would, only it was available 24/7 (well, excluding Tuesdays for 1/2 the day). That aspect has been lost for a long time now. I believe it's because Blizzard missed one key element in their various iterations to gameplay over the years. They introduced LFR, which is a huge win for players who want to see content but aren't hardcore. They homogenized so much between classes that you could roll anything and largely be successful at whatever quest or chore you were pursuing. Another win, in my book. But as with all change, there is some level of tradeoff. They gave casual players accessibility and hardcores Mythic/Mythic +. Alt lovers could build an army of 'toons. Transmog makes those who care look great. Finally, maybe WoW has simply run its course, as all things do.
Still...how about adding a buddy system? Let's say you enter an instance or even LFR as a pug. Everyone you run with is somewhat "linked" to you, much like the old days of a Raid ID was. In a given period of time, or possibly indefinitely, when you group with that person and run instanced content or even WQ's, you gain a multiplier of some type. Could be for loot ilvl, improvement to a key stat, or simply gold for finishing said content. This would likely encourage more regular grouping, and some people, in spite of themselves, may even forge those in-game friendships that help enliven the game itself.
I've been off WoW for a bit recently returned, so maybe they already do this and I'm just lagging. Or maybe I'm thinking of how I sometimes have to manipulate my kids into decisions that they would have rebuffed independently but come to appreciate later. Either way, I have always believed that WoW wasn't great because of WoW; it was great because of the people inside the game.