I'm all for randomness or random events but keep it in the overworld and make it so that everyone gets a chance to experience it, it can be switched up for different players for all it matters as long as the rewards are in the same amounts. Everything in a MMO needs to be able to be open to everyone as long as they put forth the effort to do so, locking someone out of something because they essentially didn't get the number to get that version is going to piss a lot of people off, even more when you give the random even more or better loot.

As long as these things are followed then anything could work.