So what type of dGPU would you like? A high-end one, perhaps? We know that won't happen - costs too much, generates too much heat. If you want that, well, I want a $1000 supercomputer but I ain't getting that either.
So perhaps the same dGPU that's in the current 15" MGP's? That would be the AMD Radeon R9 M370X. The thing is that if you look at the Skylake mobile processors (it's pretty much a given Apple will continue to use mobile processors in the Mini), the 6770HQ is a 2.6Ghz quad-core with the Iris Pro 580. According to the following site:
http://wccftech.com/intel-iris-pro-graphics-gamers/
the Iris Pro 580 has a benchmark of 1152 GFlops. The Radeon has a benchmark of 992 GFlops. (OK, I have no idea what configuration of the GPU's generates these numbers - if somebody has specific computers with specific benchmarks, we'd all like to see them.) So, if you're looking to get something like 15" MBP graphics performance, hoping that Apple uses the Iris Pro 580 in the next Mini is a far more realistic goal to dream about. The Iris Pro 580 is supposed to be able to drive a 4096x2304 monitor at 60Hz using DisplayPort, for HDMI it's 24Hz.
I know the popular urban myth as to why there's no quad-core 2014 Mini is because Apple felt it would cannibalize iMac sales, but another reason that I saw floated was that there was there was no common socket between the dual and quad core mobile processors. I don't know if that's true because when I looked at the 2012 Mini processors, I couldn't find a common socket between the i5, i7 2.3Ghz and 2.6Ghz processors that Apple used. Anyway, there is no common socket between the Skylake quad and dual-core mobile processors (so far released). But Intel does have quad-core i5 processors without hyper-threading which uses the same socket as the 6700HQ. There's two processors - the 6350HQ 2.3Ghz with Iris Pro 580 and the 6440HQ 2.6Ghz with HD 530, which has a benchmark of 442 GFlops.
One reason to hope is that the 6770HQ has a "list" price of $434, the non-HT quad-cores list at $306 for the one with the Iris Pro and $250 for the HD 530. The 4278U processor which is the i5 2.6Ghz processor in the 2014 Mini lists for $315.
EDIT: Re-reading the OP, the title would suggest a dGPU like the 2011 5,2 Mini whereas the poster wants a new line of Macs where one could put in a PCIe dGPU of their choice. I think that's as realistic as Apple re-introducing the internal optical disc, as one poster in the MBP forum wants. A USB-C port with either USB 3.1/Thunderbolt 3 where one can attach a GPU is as much as can be hoped for.