I do like this concept and to expand on teriyaki's idea, I wouldn't go so far as to say 7 world wonders at once, the rarer the better and I agree they would need huge resources and rare resources at that, but I don't think they would be allowed to be built on tribal land, as to make them destructable by other players without having to go to war with someone. And I think that they should provide some bonus to the land around which it is built like say trees within 200m's grow faster or grass within 100m grows faster and not like in a day, maybe shave off 1 day of the normal timer. So it's not a huge bonus and it's not a needed bonus more of a prestige thing to have built it and protecting it.