Why you want to do this? How much processing power you are looking for?
you can buy a used 2009 Mac Pro with less than $1000 (dual processor model), and can be maxed out by two X5690 to 12 Cores (24 Threads) 3.46 GHz (Turbo 3.73), which cost you another $650. The CPU architecture is old, but the processing power is not, only the max out nMP can barely beat this cMP's multi processing power. Of course, if you only need the single core performance, go for the new iMac.
Otherwise, I don't think that you can get any other Mac with this processing power but a lower price. (For info, a used single CPU 2009 Mac Pro cost <$700, and the W3690 <$300)
On the other hand, the old Mac Pro actually can use some newest hardware which is not available on any other Mac yet. e.g. The GTX 980 graphic card.
So far, the cMP (NOT the nMP) has the fastest storage option. A $10 PCIe adaptor can make the cMP use some SSD that has 1200MB/s read and 1400MB/s write speed. And most likely the cMP will be the very 1st Mac that can use the upcoming super high speed SSD.
Thunderbolt is not available on the cMP, but just $20 and then the cMP will have USB3.0 capability (or 3.1 later). Moreover, Wi-Fi ac, BT4.0, SATA 3.0 ... are all available on the cMP.
Yes, it's old, it's from 2009, but due to it's upgradability, it's hard to say a properly upgraded cMP is "too old".
However, I can't persuade you that it's not too big
Anyway, since I don't know the answer of my 1st two questions. So, that I can hardly give you any valuable suggestion. But I guess the 2009 Mac Pro can actually do much more than what you believe.