The problem, is making content last, without low drop rates, without stagnating your character growth, without becoming obsolete.

IE, impossible.

People get tired of running multitudes of dungeons, yet we are supposed to have something similar that lasts years? Tomes are evil, but how do we get gear otherwise? if it drops every time, then you finish gearing and have nothing to use that gear for. if it rarely drops, then you are just doing the same thing as getting tomes, only with no rewards 90% of the time. 10% drop rate, up to 8 people in the party want it. Would turn into the minions from dungeons all over, but with gear.

People always say open world dungeons would solve the problems, but how? You are still going to have to grind it out, just in a different way. It would essentially be open world diadem, but with mobs dropping keys for chests? something like that? and then 99% of the time, you get grade 6 dark matter.

Hooray.