Ryzom is pretty old at this point, the playerbase was fairly big by 10 years ago standards, at the time. I pointed it out simply because of the player made content, which never took off too much while i was playing it.
There was a point early on in the WoW days that a lot of people were talking about Ryzom, it being like WoW but better. Same old story we hear all the time these days, but back then WoW wasnt so gigantic and games that had the polish to compete with WoW were just not around yet.
Anyway, you never can be sure what it is that makes an MMO gain enough momentum to really gain popularity. Look at Haven & Hearth, basically a fairly primitive game with weird mechanics and strange UI, but for some reason it really gained popularity. I leave it to others to speculate why.
And of course, it doesnt take a million people like WoW to consider yourself successful, but if you could get as many players as Haven & Hearth did, you would be pretty successful I think.
I think to really hit it, you need something super special and different. It's a cool idea to allow people to build their own islands but as I suggested before, I would love it if you took it one step further. Allow island owners to be a sort of GM on their island. Let them breathe life into it the ways GMs do in Neverwinter Nights. Let them jump into NPCs if they want and animate them as they would a player avatar, or let the NPCs follow their routines normally. Let the island owner spawn special encounters and monsters to spice things up and add replayability and a dynamic feel to their islands. Make some sort of announcement system where island owners can announce when they will be on and running special events on their islands.