Intel has some devices code named "Rocket lake" on their roadmap which appear to couple 14nm CPU die to a 10nm GFX die. That may be the first Xe ( and perhaps labeled Gen 12) graphics. But Intel doesn't have to wait until 2021+ to get to the newer graphics. Just not integrated on eh same die, but could be in the same package.
The mini logic board may not have space for an incrementally bigger CPU package if Apple keeps the Mini's dimensions cast in stone. This Rocket Lake has turned up in the U class processor swimlane on some mobile roadmaps, but I think there is a S class also. Intel did a x86+AMD GPU mash up. It is extremely likely once they have their own discrete GPU. that they will do the same thing with their own.
10 cores in the space of a mini I suspect will base clock that very few will like if implemented on Intel's 14nm.
I get the feeling that the form factor is set in stone given the likes of MacMiniColo will be placing orders en masse at regular intervals and I imagine they have some clout with Apple.
It could still be easier to access in my opinion.
That doesn't mean that future Minis won't return to mobile CPUs (with better GPUs) if the situation suits them and they wish to do such a refresh. Desktop CPUs tend to go out of service quick quickly with Intel compared to their mobile offerings and Apple would need to explain why the CPU grunt would suddenly go downhill compared to the next generation.
The AMD GPU G series chips were reviewed as running hot (and power hungry) at the time so that must explain why Apple have never used them in the 15" laptops and stuck with separate Intel + AMD GPU combinations.