...
When the right components are available, and the initial sales rush for the 7,1 has cooled off, I fully expect a refreshed MacMini to fill some of the "hole" in their lineup, perhaps Q2 2020...
The Mac Mini refresh is probably relatively highly decoupled from the Mac Pro release. If Mini refresh takes until Q2 2020 then ....
"..I asked an Apple source last fall why it took so long for Apple to release the new MacBook Air. Their one-word answer: “Intel.” ..."
https://daringfireball.net/linked/2019/05/01/cook-maestri-intel
The Mini is either waiting on an 8-10 core 'Comet Lake' solution or a 6-8 core Ice lake solution. Doubtful that it will get a major volume increase and a discrete GPU. The Mac that the Mini more likely would move in an orchestrated pattern with is the iMac. If Apple bumps up the graphics with Gen 11 iGPU ('Ice Lake' ) then they'll move the discrete GPUs of the iMac 21.5" more to keep the gap.
In theory, they could re-introduce the cylinder 6,1 with parts appropriate to the thermal envelope and sell it in the $2-4K range - but that seems extremely unlikely. IMO the 6,1 design was a fail because it was supposed to serve as the centerpiece of a Thunderbolt extensible workstation. As an upgrade to the MacMini, it might have been a winner - small, quiet fan, much better cooling architecture than the Mini and lots of ports. Yes, I realize that the cost of developing the cylinder design would have been hard to justify for a lower ASP product - but now that those costs are long ago amortized...
the Mini probably isn't going to become a "headless iMac". Apple has already thrown the Mini into the > $2K 'hole'. ( BTO top CPU , 32GB , 1TB SSD , 10GbE is $2,399 even sliding back to 512GB SSD is still over $2K ). mid $2K Mini plus a eGPU is in $3-4K range. There is already overlap with the iMac. Apple won't do yet another desktop overlapping in the same space.
The Mini and iMac have been firmly pulled into the $2-3K space. The iMac Pro is frimly in the $4-6K space. They arent' slot boxes but they are 'hole' fillers.