Well, the site is really based around the avatars so, in my mind the game would center on that.
The story would be, at least on the surface, that you're a regular old citizen in the slums of Aekea (I say Aekea cause...the other towns just seem too damn happy and clean to have slums).
You've always wanted to better your life, and it just so happens that the gods of Gaia are holding a contest. The prize: An angelic halo. If you got your hands on that, you'd be set for life, even help your friends and family a bit.
The details of this contest were sent out in...a monthly letter ;D It wasn't very specific, but it gave you a map and clues, things of that nature to get you moving along.
But along the way, you realize there's more going on than that. People joining in this contest, have started disappearing...
The real backstory would be either...just elitists elminating the poverty line of gaia, trying to cleanse it or... you know, something just completely evil and a*****e-ish like that. I mean it would suck if you were just chasing after a halo, that would be stupid.
In your journey though, you'd be able to buy new clothes and things like that...hence the avatar part, or buying potions to change your form for various reasons...
Not sure what I would have for a concept for the game. It would depend on what it was if I would play it.
I had the neopets game and that game was really dumb. Even though i still go on neopets from time to time. Didnt even finish the game.
It would need quite a bit to hold my attention. Im still doing misc quest and stuff with one of the expansions in skyrim. Harvest moon is another game type i'd like. As long as there is some depth and free to do on your own stuff in the game.