DVR, though my reasoning is a little warped.
PDA I would like to see Apple make a PDA but it is an area where even experienced makers in the market are struggling right now. Maybe Apple will hit upon some usability combination that others have missed but at least in the U.S. PDAs are passe. Japan would be a good target market for this but traditionally Apple has always started with a U.S. based market.
iPhone Convergence devices do not particularly interest me. Eventually it might work but a PDA needs a bigger screen than is feasible on a phone.
Tablet I can see this for the Powerbook in 2 or 3 generations but I am not interested personally. I am very keyboard centric and the Tablet PC is anathema to the way I work.
DVR Apple has experience in the media market. They have the engineers and software developers who know how to make something like this work. It is sympathetic with Apples other stated goals and focus (unlike any of the other devices above). And finally it increases the visibility of the Apple brand into another market segment, namely the home entertainment center. It works on all levels to make Apple a stronger company and extend their reach.
Once they have a DVR though, I would like a PDA.