A Topalov–Kramnik final is theoretically possible and would be quite entertaining.
Indeed. I was thinking about that in Sinquefield Cup. What would happen if they were either in the same team (Team Randy) or playing against each other as Randy's team vs. Rex's team. It'd be awesome/awkward. 
Many top players failed to win their first game against much lower rated opposition: Grischuk, Giri, Gelfand, Jakovenko, Tomashevsky, Radjabov, Adams, and Kamsky even lost (even if I suppose he no longer may count as a top player).