There is a fair bit of empty room inside the current 2018 case and the Apple Silicon system board will take up less space than the Intel one, plus there is the volume of the internal PSU which will be removed. So even making the depth less (with the same length and width), it should be able to keep a 90W TDP M1 MAX cool enough.
One of the readers made this logical comment
So you really can't dream up a high end Mac mini until Apple gets the upcoming 27" iMac successor out.Tech In RL
I’m going to disagree that the upcoming mini will not have double-stacked M1 Pro/Max chips. Apple seems to be more consistent these days with their lineup. The current M1 mini has basically the same power as the M1 24” iMac. That follows that the upcoming mini will have the same power as the upcoming iMac Pro 27”.
That's a logical point of view.One of the readers made this logical comment
So you really can't dream up a high end Mac mini until Apple gets the upcoming 27" iMac successor out.