Ok....the case. I have some ideas....
1: Drop the PSU down to the bottom. The top-mount is a relic from the days when cases didn't have dedicated exhaust fans. It now only serves to pump out heat near the CPUs, and bring the machine's center of gravity way higher than it needs to be. Not that kicking a tower over is that big of a deal anyway, but putting the PSU in the bottom changes tipping it over from "maybe, if it got ran into by someone..." to "You'd have to be trying on purpose"
2. Move the CPU tunnel up to the middle. Then, the cards go up top, where you can install some top fans to add airflow to those. Since you no longer need a fan in front of the cards, you can put the optical drives there, then shuffle the HDDs to bottom, where there'd be room for 6-8 of them.
"It's timeless..." yes, but this is Apple. It wouldn't be the first time they re-did a Mac's chassis for just for the sake of a new look. A lot of people said the old MBP/PBG4 case was timeless and couldn't be topped/shouldn't be replaced.
3. It needs to be bigger. If they pulled it out to about 24" deep, there would be a room for at least one more HDD without even changing anything else. The current machine is 20"x19"x8". That is big for the family desktop, but what's claimed to have "tons of room to upgrade" and be "fastest ever"....IMO 24"x24"x10" is better for a "tower of power.
Ex:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811133056
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119138
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119103
4: Too many hard angles, too much silver. It's starting to not look like it belongs with the other Macs. You can tell there's been some changes to it's siblings that it hasn't had yet. The "new Mac look" is rounded corners, black accents, tapered edges, etc.
5: Everyone else is doing it! Well, more seriously, the Macbook (iBook), MacBook Pro (Powerbook) and iMac all went Intel keeping their PPC looks, but were later updated. Apple did this for all 5 machines to help show "It's the same Mac, just faster". Now that everyone was accepted the "Intel Mac" as being a "real Mac", they started freshening up the designs. The iMac went first, we saw black, aluminum shell, glass where applicable, and very, very few hard angles. A lot of curves. Then the MacBook Air came out, all tapered and with black keys. Then we got the unibody notebooks, more tapering, same black keys. The new ACD, same thing. The new Mini is probably going to look like a small, fat (closed) MacBook. That leaves just the Pro looking like the PPC version. It wouldn't fit.
I really have no ideas on what the exterior could look like, but those are my thoughts on that.