It'll never be in a low cost Mac Mini platform - I think that the Mini as we know it will be based around the 15w CPUs going forward - the i5-8250U is looking an obvious contender.
However, on the assumption that we're about to see a modular Mac Pro at some point and some people think it starts at $4k there should be some research on for the $2.5k headless market that is currently covered by the base 2013 Mac Pro. Surely Apple would be happy with people spending serious iMac level money on a product?
Would it be inconceivable to see an Apple version of this in a more elegantly designed case with nice cooling? I just doubt that it will come with the RAM and storage slots that home brew users would hope for. The i5-8305G from the Hades Canyon would be a great basis and people would be looking to compare with the 15" MacBook Pro that this hardware ends up going into.
If sold at Mac Pro prices I'd expect such slots to exist but be pre-filled with 16Gb RAM as SODIMMS and at least 512Gb SSD on a PCIe M.2 slot. That way any replacement of RAM or storage is for maintenance purposes for pro users rather than servicing people hoping to avoid Apple tax with a bare bones unit and their own upgrades.
If Apple did this they'd be free to really go to town with the modular Mac Pro in terms of pro-level Xeons etc.
Mind you, Apple studiously ignored the Skull Canyon before this...