I'd like them to make the Mac Mini more expandable and offer more expensive, feature-rich models that help bridge part of (well, the lower part of) the gap between the Mini and the only other desktop system they sell, the PowerMac. Make it easier to open and the parts, easier to replace.
I don't know if it's even technically possible, but it would be great if, just as independent manufacturers are now offering USB and FireWire-based add-ons designed specifically to fit underneath the Mini, there could some day be a way to expand the computer's capabilities through the use of similar 'modules' the way normal desktops are expanded via PCI. So instead of just add-on USB hubs and external drives, there could also be, for example, add-on video cards. I don't know what kind of an interface that would require though.. that couldn't be done with FireWire, right? Or could it? What if there were some kind of connector on the bottom of the Mini?
If it were possible, then the 'high end Mac Mini' could just be a regular mini with a faster HD and processor, a DL (or BlueRay, eventually?) drive and an optional plug-in module underneath it to house additional PCI slots. Still 'mini' compared to other desktops, but now as versatile.
**DISCLAIMER** Yes, I realize the Mini as currently conceived is a "low end consumer machine", and that, because Apple at this time does not deign to make it anything more than that, I should be ashamed for daring to doubt their marketing wisdom. But the need for a mid-range, non-all-in-one desktop system is real, and given the choices I'd rather see it as a souped-up Mini tower than a stripped-down PowerMac.