I see where this is going...
There is a need for a $1000 - $1500 mid tower-size Mac machine that has an upgradable video card, support for multiple video cards, support for external monitors (choose your own monitor, add your own), as well as Bays for internal Hard Drives/SSDs, and at least four internal RAM slots (I know the iMac does have this and supports 32GB of RAM on current 27" models).
The key is providing a highly upgradable Mac machine, customizable internally, at a significantly lower price point than the ~$2499 entry point of the Mac Pro.
Years ago, the Power Macintosh 7000 series was this machine. It was the "mid-level" upgradable PowerMac, that had an upgradable processor daughter card, RAM slots, PCI slots, and internal bays for hard drives, etc.
I am talking about the 7500/7600 and the 7300 here. The 8500 and 9500 were more pro-level and very pricey, like the Mac Pro.
Such a machine does not exist in Apple's lineup at the current time, and has not really existed since the Power Macintosh days. Some of the lower end PowerMac G3s might have been considered "affordable" and were indeed upgradable, as the iMac was pretty much a closed system (and still is)
The Mac Pro is a great machine, but many people don't have the need for > 4 cores, 2x DVD drive bays, 4 PCI slots (some would be happy with just one or two for a video card)...
So a Quad-core Ivy Bridge midtower-type Mac machine with two PCIe slots, a decent graphics card, and expandability to 64GB of RAM, an upgradable processor chip, and two internal 3.5" drive bays would be enough for most people. I think that's the machine people want....and at a $1000-$1500 price point, running Mountain Lion, I think they would have buyers.