I think that given the actual internals of the G4 iMac and the G5 iMac, a newer style cube, aka headless iMac could be feasible at a decent price point.
Give it all the features of the current iMac, but without the monitor. Of course, people would howl with rage at lack of PCI, user upgradeable AGP, etc. But it would be beneficial as a product matrix model to sell a G5 cube, for 500 less than the G5 iMac.
When the G4 cube debuted it was a manufacturing nightmare. It costs a lot to make those cases. With current design themes, these could be aluminum style and look nice and cost much less. Who knows, maybe this is in the works.