Yeah, the thinner it is the harder it will be to make the design radically different. Well, actually it won't, but if you want to stick to traditional Apple design it will be hard. The change would pretty much have to be different materials and colors. Perhaps a more rounded shape.
I would rather have one of two things:
1) This may sound weird but I'd rather have a "thicker" iMac with more style. Just look at the histry of the iMac: First the revolutionary all-in-one egg Mac. They made you say, "How did they do that?!" Then they went to the swivel-neck Mac and again made you say, "How did they do that?!" Then they went to the panel Mac and made you say, "Where did the computer go?! / How did they do that?!" If they were to go to a slimmer, aluminum, chin-less Display Mac, there won't be a "How did they do that" reaction. It'll be more like, "That's is?"
The other part of this is capability: If the iMac keeps getting thinner, it'll never have more capability than a MacBook. Personally, if I were to accept the drawback of not being portable, I'd expect a trade-off: more power. That's something that can't come with a slimmer design. The iMacs of yore used to be just one step behind the PowerMac. That's part of what the iMac so amazing. Well, the current iMacs are three steps behind the Mac Pro. I used to be able to buy an iMac and be a Prosumer. That is no longer true. The iMac needs more power, and I'd be willing to sacrifice thickness for it. It's a desktop anyway! Why are we caring about how small it is?
2) They should go the other way and make it a lighter-powered convertable.
Option A: Give it a base station with a nice GPU, HDD, extra RAM, USB ports (for keyboard, mouse and others), optical bay, and a PSU, all sitting on a nice clear acryllic stand. The display detaches (and becomes multi-touch capable), the unit complete with mo-board (using integrated graphics), some RAM, 16GB SSD (enough for OSX and some free space), Wi-Fi, and a battery for a good 5 hours of detached life. A good ol' tablet is you will.
Option B: Just a multi-touch detachable display, Wi-Fi, and a battery, loaded with enough guts to act as a "remote desktop" interface to the base station.
Out of the two choices, I'd say 1 is more likely but I'd love, love, love to have 2.B.
mmmmmmmmm detachable display....
-Clive