In addition to upgrades of all of the existing lines, I would think that for Mac's 20th anniversary, Apple would introduce something at the very top of their line (like the original '96 Apple 20th Anniversary mac (featured in Batman Forever!)).
A sphere would be pretty cool. Apple may also do something that is a page from their past, which is an all-in-one unit. How about a 30-inch cinema-display LCD mac, which would be like 3 inches thick, or 4 inches max, and have the motherboard, expansion slots, ports, etc. all behind it, in one beautiful rectangle merging the tower and the display in one, that you could stand on your desktop or mount on the wall. You could sit back on your couch and surf the web, or watch a DVD, controlling everything from your wireless mouse and keyboard. No wires anywhere! Just Bluetooth and Airport!
Of course it would be dual 3Ghz G5....
And the commercial during the superbowl would be the exact same as the 1984 commercial. Except at the end, when the hammer goes into the screen, you pan out, and realize that the commercial is playing on the new mac mounted on the wall of the living room, then pan out some more, and you see the guy on the couch with the keyboard,
editing the 1984 commercial with Final Cut, and laying a new track with Soundtrack, and with all the other iApps going on in the background -- to show-off all the things you can do so easily, only with the power of the mac.
Just a thought to Apple and Chiat/Day....