Have you read that interview to tetsuya nomura about advent children , he was asked about where AC take place and this was his answer :
PM: Where, specifically, will Advent Children take place? Will it be somewhere from the original Final Fantasy VII?
TN: It's hidden in the logo

I have seen the logo loooooong time and all that I could find was like a siluete of midgar on the sphere. Maybe the little flower at the up-right can mean something about aeris, or maybe the same squares in wich Cloud and sephiroth appears in the promotion, when they are like in a fusion. Have you find something more insteresting than my obvious comments ?. Write something in this topic while the movie is being made.