Video game design 101 is to make it so you could find that special item at any moment, to keep you playing. See: borderlands 2 or WoW.
I don't need "finding a special item" to keep me playing. As far as I'm concerned, either the game is interesting or its not, period. That's probably why I don't play either of the games you mentioned.
In fact, I want just the opposite: I want to know what the standards are and whether I qualify, the same as I can look at any world during jobs and find out where I could probably get a job and where I have no shot. I do NOT want to play guessing games as to when I'll get in, at least not on a large scale.
As for making the world full, I don't know that this is necessary, and certainly not to the extent that the "Legends" world should just allow anyone to fill the last few spots.