Not sure why Apple would go with the Y series CPUs for the Mini though the limit on size is partially down to having to use 2.5" hard drives for the lower end SKUs.
The U series MacBook Air class 15w CPUs would be fine and, yes, people who really want to get serious compute power on the cheap would then connect up a 2018 Mac Mini with Thunderbolt 3 ports up to an eGPU (which combined could double the cost of the Mini).
This is why I would advocate the
i5-8250U as the basis of a 2018 Mini. At the moment, bitcoin miners are artificially inflating the price of PCIe GPUs - this year isn't an ideal year to buy.
Sonnet Tech are currently selling
eGFX breakaway boxes (Approved by Apple) with (for example) AMD Radeon RX Vega 64 8Gb card for $1299. If one of these boxes could charge an 87w 15" MacBook Pro then it could theoretically power a 15w Mac Mini. Note also that Sonnet sell a
base model which can handle a 15w Notebook.
This releases Apple to make another sealed unit Mini with 2 Thunderbolt 3 ports, one for the GPU, one perhaps for a storage array.
If the 2018 Mini uses the same case as the 2014 but has a more sophisticated cooling system we could see the CPU in turbo mode for longer. They probably don't even need to improve the cooling system to be fair as that case is capable of cooling for a 45w TDP.
All of this of course releases the semi-pro user from waiting for a better option. They purchase a 2018 Mini, add external AMD GPU, perhaps external storage as they see fit and get on with their life.