Well it would not be very "mini" if it was in the cMP case.
And yes, I know people want an iMac 5K without the (5K) display, but Apple is not going to offer a headless Mac that directly competes with that so while yes a cMP case would allow 6/8 core i7s and dual Radeon Pro GPUs, it's not going to happen.
I do think if the Mini is updated, it will get quad-cores again - if for no other reason I believe Intel is going to make that the minimum core configuration in the near future now that we have Kaby Lake Refresh quad-cores with 15W TDPs.
I think our last, best chance to see a new Mac Mini is on the 12th because of the Kaby Lake Refresh CPUs launching. Their max TDP is 25W which is a bit less than the Haswell's currently in the Mini and Apple has three dual-core and three quad-core models to choose from. So they really need to do nothing to the case except swap the TB2 ports for USB-C/TB3 ones.