games need to make more content faster, but it just isnt enough to satiate people.
Something that I've noticed plagued warcraft was a belief that if someone that isn't you cleared content, that content was done for. They join a guild that has 12/12 bosses down and they think "oh, guess there is nothing to do".
MMO's cater too much to a group thought. Players need to be told they are more independent, and base the games around building your character, not what stupid accomplishments you do in a group. That should be a secondary objective as you socialize.
The whole "classes" deal is really old now too. You should have proficiencies in certain areas, and you can just get better at those and switch items. People will just make alternate characters anyways, no reason to just not make it all one character.
A problem that would come up here is the inevitable "these are the best items/weapons/stats you can use" then everyone will do that. Runescape is a bit like that for an example, but I think that may be due a bit to how ranged and mage are both the art of practically throwing money at an enemy until it dies, compared to melee which is a one time investment.
That's a game that I think people seriously overlook.
I think that also, once a game gets too large it will appeal to the wrong people who are too stuck on thinking they need to get from point A to B, instead of enjoying the world. Runescape as an example again, people say it takes forever to reach level 99, but it isnt about that, its about having fun and immersing yourself. WoW is actually a fantastic MMO for this reason, its rich in lore and a world to explore, but they're really fucking it over with how streamlined it started to become in the second and third expansions. Catering to people who don't even like WoW, just only focused on getting a piece of gear and log in only to raid, with a character that has 1200 achievement points at max level since they never actually play the game. Same thing with idiots playing unbalanced arena thinking they are actually playing the game, then complaining about it having "nothing to do". There are worlds to explore in these games, and that's what they are for, the greatest source of escapism in any type of video game Genre.
I'd hate to say it, but this is another thing that "normals" ruined. Games like that become popular and thats the only reason they play it. I don't play that star wars game, but I think its good that the game is losing those customers only there to grind and reach max level or ride a hamster wheel to get whatever there is to show. That looks like a game for real star wars fans. It will probably end up shutting down because a company doesn't understand the value of dedicated customers.
There are few games that actually keep good and steady, and the only ones I've seen have been Runescape and Eve online. The playerbase isn't what matters, but the universe you craft, and how deep those people will immerse themselves in it. WoW really is going down the shitter with this new thing, where there is no lore continuity, just whatever they brought out of their ass as new lore. It could have been done with a lot more tact I think.
now I hear elder scrolls wants to do an MMO also. The history for that series is rich enough that it could work if they abandon their current trend of simplification, or the WoW style copy trend every MMO tries to do now. Complexity of your character will draw players in and make them feel much more invested than something simple I think. Then again, you have a game like league of legends that has simple 4 button characters and only 3 maps, yet is insanely popular. But that isn't an MMO I guess.