I guess that's the downside to marketing hype. Once everyone is convinced how incredibly the G5 is, no-one will want anything less. Irrespective of actual performance.
If the Motorola rumors are to be believed, then they DO have plans to scale the G4 towards 3Ghz, but in general it's said it's at the end of it's lifecycle. The real point of the G5 is that it will scale well into the future. Where the G4 has left off, the G5 supposedly picks up. So obviously to advance, the iMac has to move to G5s sometime. Other rumors indicate Apple has been working on a redesign, which would seem self-evident if they want a G5 iMac. The heat issue is significant. The iMac is a very compact machine, not a lot of airflow in there. So, it must be redesigned for that alone. Not to mention the whole motherboard. So why not a whole new look to mark a new iMac G5 era?
The basic principle of the flatpanel iMac will presumably be carried over, the swingarm idea is just too damn convenient.
Whatever became of the fruity colors? For some, that's what make the old iMac stand out and desirable over a PC. If they could device a way of customising them with detachable colored opaque panels, so dealers could swap them over themselves and hence not be stuck with computers in an 'uncool' color, that would be great. People migh want to periodically give their Mac a new look, like they might with their Nokia phones. It adds nothing to the speed or performance (unless you believe the school of thought that speed stripes make a car go faster), but adds to the 'cool' or 'desirability' factor that people may pay a premium to get. Preferably they shouldn't HAVE to pay a premium, though.
Personally, although I know it's unlikely, I'd like to see the Cube resurrected in some form, which would be separate from the iMac. Steve never ruled it out if the time was right. It would have to be somewhat bigger, needing custom-built cards makes upgrading expensive, and leaving some room for expandability like RAM wouldn't hurt. I've seen a PC that's borrowed the Cube look, it's much bulkier, but pretty cool nonetheless. Certainly more airflow is needed. People didn't like plugging things into the base, but I'm not sure how that might be prevented. Price killed the cube more than anything. If you got everything including the screen for what they asked, it would have been tempting. But a flat-panel to boot, which at the time was much more expensive than now...
We need reasonably-priced Macs, or Apple won't increase their marketshare. Lots of people would LOVE to get a Mac, but they won't with the current price structure. And, of course, people have to KNOW about them! Better advertising and marketing. Macs are not bloody BMWs. Most luxury car makes, like Rolls Royce or Jaguar, have been bought out by large makers of small consumer cars, like Fiat etc.