Replacing the Premium would add at least $100 to it's price, and maybe as much as $200. MS would be better off sticking to Core, Premium (HDD+Wireless etc.), and HDDVD (Premium+HDDVD internal). Alternatively just have Core and HDDVD versions but with packs for peripherals. That way users can pick the DVD or more expensive HDDVD versions, and buy packs of HDD and whatnot if they want them, or buy them individually over time.
Agreed, there has been the "360 AV" version rumored for awhile now and that would slot in nicely there (and its rumored to be black to boot).
It would make sense to offer this option as it would give them an alternative for people buying a PS3 for its 'all-in-oneness', they dont have a product that competes 100% in that space.
Again, my only misgivings are the HD-DVD read speed, otherwise i think this is a total no-brainer.