I don't represent all players in this of course and much of this will be personal. However, I know there are a lot of people out there who are former WOW addicts who will feel similarly to me on many accounts. I am not one of the bittervets who complains about everything after vanilla being a downgrade - lots of things were great, some sucked, and I just don't want to play like I did in years gone past for both reasons that are internal to the game and reasons that have nothing to do with it. The draw required to get me to resub to a new game is going to be higher than it was back in 2004 for sure.
There seem to be two things I care about. One is that I want all my friends to be playing. I want to log in and see all kinds of familiar names, I want to use ImaginaryMMO as my social platform, and I want to be surrounded by all those people I remember from days gone by like those who somehow turned from teenagers into real people while I wasn't looking. So yeah, make something so awesome that everybody I know loves it. Good luck with that and sorry for the useless advice, since if you can get everybody I know playing you are probably already taking a dive into a pool of money.
The second thing I want is a world rather than a story. I distinctly recall the first while of playing WOW and the thing was that the world had all kinds of interesting corners to poke about. There was this place called Scarlet Monastery that was super far away and mysterious and our quest to clear all of the enemies out of it was a bloody adventure. The story was mine to write, and it could include a trip to Scarlet Monastery to destroy those dastardly fanatics... or not. Clearly there were all kinds of terrible problems with the world and quest design back at launch but it is possible to fix those without making the world into a single hallway along which the characters run.
That isn't to say that I glorify the inconvenience of the old days. Walking forever to get to a dungeon is frustrating and things like summoning stones and dungeon finder teleports eliminated a lot of tedium. The trick is that I really like it when a game is complicated and hard and optimizing it is actually a real thing. I want to find the perfect path through the quests to do them at maximum possible speed. It feels good to figure out the routine, to find the points where the game can be made to bend to my will.
I don't want a novel. I want a choose your own adventure. I don't want to watch, I want to play. I kind of doubt that any game will recapture me in this way though - they all seem to be novels these days. I suspect until there is a major revolution in the design of MMOs they will not be able to suck me back in.