I think there would be a market for a hub (with the lack of usb/firewire) and hard disk. Many people will buy the mini with only a 40 gb disk, which is also pretty slow. As it is targeted to iPod users, many customers will have 20-40 GB capacity for tunes alone. Of course not everyone fills their iPod, but even so... And with a review on Maccentral stating, i think, that the software on the mini takes over 13 GB, there already isn' much room to play around with and space would run out fast, creating a quite healthy market for an apple branded hd component.
Apple has possibly made the design like a flat slab on purpose, to make it like a new size of stackable components that will replace a hi-fi stack in the living room. The mini already has a dvd player, Apple could quite easily make an iPod station, which has high quality audio circuits in and sinks with a iPod and works as a music player in its own right. connecting it to an mac mini will allow the user to be able to download songs from the itms onto it. Asteroid already looks like it is designed to stack with the mac mini, although the ports on the top would have to move, unless it was the top unit of a second stack, such as how them mini separate systems are often stacked in two stacks of two. I wouldn't be surprised if Airport is redesigned in the mac mini style, so that if you have multiple computers it could be stacked under the mac mini closest to the broadband ethernet jack or just match it's style anyway. The current airport looks kinda dated now most new white apple products include aluminium as well except the iPod, and the current design has been around a while.
To me it makes marketing sense to make add-ons that match the mini. Users are more likely to buy the apple styled add ons than third party if they don't cost too much more, the mini is a low margin product so selling accessories is a way to maximise profits (as they have learnt with the iPod and its ecosystem) and it could even push users of other macs to buy a mini if they already have a mini styled airport, breakout box (asteriod) for garageband and a hub for their existing mac..well because its only $500 and it would fit in with the rest of the stuff and it would always come in useful for something.