As the Mac Mini is effectively a laptop without a screen or battery, I would like it just as a laptop would be without them, a keyboard with the computer built into the underside.
Computers such as the Commodore 64 have done this in the past, although a Mac Mini could be much thinner. It would be a great space saver as the footprint is contained within the keyboard you have anyway. Plus the long thin shape would be better able to dissipate heat. And components would be more accessible, so the hard drives could be made user replaceable.
EDIT: Giuly's post only appeared after I had composed mine. Our ideas are fairly similar, the difference being mine doesn't include the touchpad, allowing a much smaller device. I prefer it this way as I hate reaching over the touchpad to type (my keyboard is on the edge of my desk).