I was reading another blog entry (Lccf's "Missing Characters in the FST") and Walt was mentioned. After thinking about it for a while, it would be interesting to see Michael without Walt in the sideways. As in, without the Island, Walt does not exist. Just like with the Island, David doesn't exist.
I don't know if the David-Walt thing would mean something or what ramifications that would have, but I know they can't show Walt in 2004 (since Malcolm David Kelly doesn't exactly look 10 anymore).
This isn't a theory. I just think it would be interesting.