I too am interested in this. While I would personally like a modern setting I wouldn't mind futuristic either depending on how far away from the present we're talking. As for disease, I think it should be prevalent enough to have a major impact on the population of the combined world and it could make for an interesting obstacle. Maybe we could explore how communicable disease from one world affects those from the other. I don't really have any idea as far as setting goes but I think it will be easier to choose once a time period has been agreed upon.