Here is my design for the new iMac. It's not spectacularly different from the ones they have now, but I doubt that they will do something completely different. They learned their lesson from the Cube.
I also included some suggestions for new colours, although I wish you could get custom iMacs, like you can get custom bottles of Jones Soda.
Ebony (Black) would replace Graphite and Sapphire would replace Indigo. It's my understanding that Graphite and Indigo (Blueberry) are the most popular iMac colours, so they would have to have colours that come close, and Apple hasn't had a black desktop in years.
I have included 2 patterns. I think that patterns were a great idea, it's just that Flower Power and Blue Dalmatian were too loud for most people. Ivory is just subtle streaks of beige and Opal is just delicate swirls.
The other colours I suggested were silver and gold, which are not actually colours, but styles. Silver is the same as the current iBooks - metallic silver looking plastic embedded in clear plastic. Gold is the same, but with a (tasteful) gold colour.
The cheapest and slowest model would come in Sapphire and Black.The mid range would be Ivory, Opal, Sapphire and Black.The Fastest and most expensive models would be the Silver and Gold.