I think you see a Bush-McCain rematch in the primaries, especially because it's a very close election, and Bush will have 4 more years of being a well-regarded and seasoned Southern sorta-evanagelical governor. It's hard to say who comes out on top of that one. I could see a losing W going for Phil Gramm's senate seat in 2002, and he'd win handily. He was an extremely popular Texas governor, and for good reasons.
I don't think 9/11 is a given. A new Administration is inherently a little more vulnerable to an attack due to things slipping through the cracks at transition time. Gore won't have the same degree of transitioning since many appointments may be holdovers. Enron/Worldcom/Global Crossing definitely is, and huge swaths of the stock market were in the same place Bitcoin was earlier this year, especially in terms of the tech stocks and nonexistent earnings, so you've got fraud and an earnings bubble.
If Gore wins in 2004, then Greenspan's extreme interest rate drops will likely lead to an asset bubble due to the cheapness of borrowing. If that happens, then, ironically, Senator George W. Bush could come in with a populist Conservative message that takes its pages from Teddy Roosevelt and storm 2008.
I think it'd be extremely hard for Gore to win in 2004, simply due to voter fatigue with the same party.