What you're asking for is actually for a "G.U. Online", rather than a ".Hack MMO"...and it would be basicly wanting to play GU, with the skills, weapons, gameplay, enemies, areas, etc of GU, except being able to choose your own character, and being able to play with anyone around the world. Not a bad idea....for fans of GU, that is. As for everyone else, it wouldn't be entertaining enough, and not because of events or whatever, but because of the way GU's gameplay is, and that involves level, skills, weapons, status and general battle system. I personally would like that to happen...but only for a few minutes, after that i would get bored of repeating the same with people over and over, and would basicly use it as a big chatroom with "pretty fields".
IF a .Hack MMO were to exist (R:1 or R:2), it would have to change ALOT of things; most of the gameplay and battle system in the .Hack series are adapted for a single player, so that would have to change completely, and you could pretty much have to say bye-bye to things like Skill Trigger, the "leader" system, the way several skills works, Rengekis, Awakenings, etc....they could only stay if they were to be changed completely, like the leader only being able to do certain tasks and giving more freedom to the team, other ways to do the Rengeki combos and finish, Awakenings being completely different, etc. Levelling up would be different, classes/jobs would have more variety, and if there were to exist something like Xth Form, it would have to be obtained by FAR MORE MEANS than just increasing your level.
A .Hack MMO, a "The World" like we see in the series, will just never exist in our world, at least not the way it is right now. Because, EVEN if it does, everyone will soon realize that the features shown in the series just not work the same way in our world, and they'll stop playing it out of disappointment...and the so-called "real fans" will just be there because it's the .Hack MMO and their dream come true and blahblahblah...but will use it more like a chatroom than anything.