Making the power supply external wouldn't work. Not to mention it would be a bad move. It was akward for the G4 cube, and the power supply would have to be powerful enough to power 4 hd's, 2 optical drives, 1 or 2 processors, and everything else. They could make quieter power supplies, but making them external would be taking a step backwards.
An aluminum design like that of the current PowerBooks would be really cool.
They could work on making the case smaller too. The current ones, while alright, are a bit bulky, and it's time for change.
Upgraded audio would be cool, but not too many people would need it. It'd be better to do a minor upgrade, but leave out the pro stuff. If they need it, they can buy it, but if Apple put it in, it would just jack up the price.
What would be cool, though, are special model desktops for different markets. A DV edition PowerMac. a Pro audio edition, a desktop publishing edition, you know. A base PowerMac design with different 'packages' that transform it into whatever you need or want, thus saving money for people who don't need an expensive audio card, graphics card, dvd burner, tons of HD space, and all that...
Of course, I imagine many pros would prefer to select their own equipment to make their Mac just right.