My idea for an Ion case mod would be a Mars Rover.
I thought it'd be nice to include a webcam on either the mechanical arm or on the Pancam tower. The actual antenna that's used on the rover would be substituted by the antenna of the on board WiFi device. I think there's plenty of space on the case mod to put several add-ons: on the mechanical arm you can mount a projector keyboard, USB hub or a USB keyboard light; the disk antenna could be a custom housed memory card reader and/or eSATA connection; the Pancam tower could house a webcam and a desk lamp with an adjustable sheath or a discreet USB connection for flash drives that you'd prefer to keep out of sight.
As far as the materials used for the case itself, the modders could use just about anything they can think of to give it an authentic look, like reflective backing on glass/acrylic for the solar panels, framed in brushed aluminum. A RAID1 array isn't really necessary but it would be nice to have an extra 3.5" bay available to add another HDD if the owner wanted to.
[edit: 08/01/09] I guess only 1x 3.5" bay would be required for an optical drive since SSD's will be complimentary from Corsair, which would open up some room for fans, which I admittedly didn't factor in to the design.