It's really hard to think of anything that would not detract from functionality we enjoy with the present form factor. Just to provide examples, Paul's awesome mockup of the "L" iMac and the other iMac patterned after Cinema Display (I liked the design as well) would not be very tiltable compared to the present iMac.
Though the idea of having a laptop that has curved bottom to match legs and make for easy resting on the lap would be interesting, it would probably mean either empty space or custom made component to fill in the space, both not very desirable feature. Getting a laptop chiller is probably more sensible at that point. And we could think about building it in only to realize that it loses flexibility in other setting such as placing it on the desk and finding it wobbly.
So yeah, Paul nailed it, it's going to be tough to think of the form factor that would improve not just in the appearance but also in functionality that the current form factors provides.
My imagination suggests that the only time we'll see new form factor is when we have different hardware... I'm thinking about Microsoft's Surface concept for example. Or some of that silly mockup of people using a holographic projector in lieu of keyboard (which could be then extended to replace the monitor). But I'm not so sure that 1) they are an improvement over the current form factor and 2) there exists a better form factor that I just lack the imagination to conceive of.
Knowing myself, I'm going to go with #2.
PS: Of all mockups, I have to say I like Shark fin in spite of it being impractical. I can see it as a great icebreaker or conversation if it's positioned on my desk and people enter in from the door facing my desk and thus fin is visible. Right now where my iMac is (in a corner deck), it wouldn't be that useful, though. But a fin Mac would be fun just to talk about.