Cosmo_ said:
it's too late..

<< PICTURE OF INTEGRATED ADDON >>
(i know it's the wrong font - sorry

)
It is nice, and I'm sure that someone will make something like that, possibly with a one or two drive 2.5" Firewire attached hard drive caddy inside.
This is where I will ask for some technical aspects to be taken into account. Namely, there isn't a magical communication port underneath the Mac Mini, so all add-ons will have to attach with either Firewire or USB, or both.
That means that they'll need the input USB or Firewire port on them as well.
So replace the TV In / Out port with a USB2 in and Firewire in port. Add a couple of LEDs on the front to denote hard drive access. Err, maybe power could be split off the Mac Mini input, but it'll still need to be powered. Thinking technically, you'd most likely have a 4 port USB hub - one on front, three on the rear. Dunno about Firewire hubs[*], but with an internal Firewire IDE interface, and a front Firewire port, that'd probably leave 2 firewire ports for the back with a 4 port FW hub. A special interconnect could be made to connect the Mac Mini's USB and Firewire port to the inputs on the Mac Mini Expansion and look reasonably tidy.
Maybe Creative could make a Mac Mini styled version of their USB sound card, Extigy.
[* edit] it seems that Firewire hubs come in 2, 3, 4 and 6 port version, at reasonable prices. I imagine that something mid-range would be used in a general expansion.
This is what it'd probably look like, although I'm crap at drawing so the connector probably looks a bit crappy. I forgot that the media card slots would use up another USB port, so I removed the media card slots, they don't strike me as a very Apple thing to include. I edited the image someone else did of course.