Yes Mac Pro is really behind in terms or refresh...they didn't make work the Gpu in terms of swapping,because they are,isn't it?Maybe a new headless mac and new cinema display with egpu slot and thunderbolt 3 could mix pro and mini together,
with a low base cilindric one with GT4 to a configuration with the cinema TB3 display...i know..its wild dreaming here but ( because of the Imac also..)..we have a lot of time from here to March..even if i see March more for Iwatch2,iphone6c,ipad air3,AppleTv-A9 parade sort of and WWDC for Mac ( Mini,pro,Cinema D..Macbook 12-2016,Macbook pro 14-16 ),then in October Ipad Pro 2-A10x,Mini5 with A9 and iphone 7,7+_A10.We will see.