Not so fast. Every once in a while we need an update on this. I think there are a lot of great ideas out there and a lot og new people joining this site. We can't disclude subjects because they've already been talked about. Besides, this is after all THE BIG QUESTION...
For whats its worth i think that it will need to be something that can top Grail/Ark without getting overtly religious (i.e. Spear of Destiny, True Cross) my money is on Eden being a setting and an artifact like the Philosophers Stone to be found therein.
finding a lost world is a bit Dinotopia isnt it?, i like the idea of Atlantis or something, a lost city but defently something eveeryone has heard about, oh well, we'll just ahve to wait, its bound to be good.