So there was Mac, Mac Mini and Mac Pro... And now we have the iPad, iPad Mini. Surely the replacement must be iPad Pro - a 60" iPad!
The main audience for a Mac Pro are high-end pros, those who charge the same price as the machine for a days work. They want the best for their purpose, and are able to leverage the power of the computer.
Secondly we have the semi-pros who just see it as an investment for a computer that will not require a full replacement a year later, so they can easily put extra cash on the table for it. Then you have the Mac-enthusiasts who just want to show off with bestest specs
or just want to treat themselves with a "luxury" computer. They will not need the power, but love it still.
There should be a server model, but I doubt that is in Apples eyes. However a pro model should have specs close to a fully featured one, but might have the extra bells really not needed.
For the rest Apple want you to either use the mobile solutions or Mac Mini/iMac.
I would love to see it as an rack type computer like the Xserve, maybe 1U or 2U would look great. I can understand that for some that form factor may not be perfect, but for most pros it should do well. And designing two different cases would most likely not make it sustainable enough for Apple. Thunderbolt should make it easy to connect it with like a hard drive rack unit or other standard size equipment.