What Apple NEEDS to do if it does a "pro" version of the Mac Mini is.... dual processors.
If Apple can make a dual-i7 processor Pro-Mini computer, I'd be in there. Doesn't need 3 drive bays: I can use the interfaces for that. Doesn't need more than what is currently there + 2 processors + more memory space. It would be an awesome machine. The graphics processor isn't quite so important, but a removable card might be preferable to upgrade in future.
The TB/USB3 allows for expandability with outside housings, so PCI slots aren't needed inside the machine. Removable video processor isn't completely necessary if the price is low enough. It doesn't need more drive space than the current models. The ports on the current are enough, as well.
Since a current MacPro with dual processors is circa $3500, a mini version like I wrote would be nice at $1500. I'd rather spend less for a lower-end dual processor Mac than buy a current single processor model or the current MacPros. A current Mac would be expected to last me 6-8 years, a mini for less than half that would be good for 3-4 years. Perfect comparable investment. I'd actually prefer a cheaper, similar, computer I can replace every 3-4 years.
What you're missing is that the two Xeon processors in a dual processor machine cost at least $1500 just by themselves. Core i7s, even the extreme versions (which can cost up to $1000 each) don't have the capability to be paired up.
David