I think the next big MMO hit is going to have to be something like a melding of SecondLife (SL) and World of Warcraft (WoW) and Star Wars: Galaxies (SWG).
Basically, you will need to create an economy that is truly player-driven with gear and items that are player driven (even down to the model designs of said gear). You need to keep individuals from being able to grind through singular improvements to unlock the next tier thingamajiggy and make them more inter-dependent. For example, if i'm a tailor, I can refine the weave pattern I use with the thread i've got to work with. I can mix and match dyes and materials at hand to make creatively shaped garments and what not... but I'm not going to magically unlock the ability to make Polyester Pants simply because I managed to assemble 100 Wool Tunics. To make said pants I'd need Polyester, which shouldn't even exist in the world until some Scientist or Researcher class actually discovers the stuff... and then some Manufacturer type has to actually produce the stuff, and I'll probably need an appropriate kind of thread to keep the stuff together.
Once you give players the ability to fabricate things (including buildings) within reasonable limits ala SL and brand things (ala SWG), you will see the arrival of player Brands and craftsmen and women of renown. To this day, I still remember a gal named Weetzie from SWG and her famous clothing shop. Her wares were so famous they were considered the pinnacle of SWG fashion by her adoring fans... all because SWG allowed you to brand your wares so all could see (upon inspection) who made that fabulous gown, smock or pant. (An internet search even turned up some archived articles that lend some veracity to my nostalgia).
Of course, you have to build limitations and checks & balances into the system, but honestly, SL pretty much solved that problem. They only allow your things to consist of a certain number of basic building blocks (they call them prims, short for 'primitives'). So you tell players, 'your crafted item can't have more than this number of triangles, this size and count of textures, this complexity of shaders (if you wanna go there).' In the case of weapons, 'you can only have these kinds and types of effects and damages within these ranges' and of course, intially, all these things are further limited by the technology level that the player civilization itself has achieved. Why not?
If you let players build a world, they'll embrace the world. If they see and feel that they are the limiting factor on the world's progress, rather than some clandestine team that releases a new patch every 3 months, then they will feel a sense of ownership, particularly if you re-inforce that sense of ownership with actual (well, virtual) ownership. Let the players own land (again, ala SL). Let them sell, buy, and trade real-estate. Let there be building developers and architects, contractors and construction companies. After all, it's a bit silly to assume that everyone wants to be a front-line grunt in the great war against the Alliance / Horde / Aliens / Klingons / Ferengi / what have you. Players could be bankers and land barons, buying and selling property, making money off of the backs of NPC shopkeepers or even other players! Build a home, watch it appreciate... then sell it to someone who admires it for profit!
Even more, if you let players destroy said world (by razing the buildings, etc.) then you'll keep 'em hooked. It lets the conflict out in 'the world' bleed into the home. There's territory to protect, things to keep sacred and safe. People will have something they believe is fighting for, instead of just a cockamamy trumped up story that you can choose to skip past when they accept that next quest. And insurance companies would arise as players and non-players alike would seek a way to damper the cost of frequent re-builds to properties that had been razed yet again.
I see real potential here...