Imagine a version of the trash can cylinder that has a curved tail, think the shape of a comma from top view. Would give it that Apple design flair they understand how to sell. The daughter case would be identical in shape, but flipped (mirrored) to create a yin/yang meeting of the tail extensions- but with some airspace to limit cooling compromises.
I fully grasp why a proper tower is a better idea than any of this - but since the chances of them going back to a big box design seem remote...
Why I think they might have something along these lines in the pipeline...
If you cover a more diverse group of market segments with a broad range of BTO - by order or by retrofit - offerings, you might realize some unit cost savings. (Small numbers, different math, but still some.)
Group 1) "I need a new one" (reasons abound)
If a single piece solution with the right BTO can meet your needs then it might be a popular replacement for the tcMP in a lot of shops. TB3 may not be the ideal bus for compute tasks like eGPUs, but it can support a ton of fast storage if you give it enough headers/lanes. This means having a lot of internal storage capacity is not as important as it was when the cheese graters with the 4x 3.5" sleds came out.
Group 2) Media Creators
Need to push some pixels (samples, whatever) and need some serious GPU resources. Want a hot rod that doesn't need a full time mechanic (read IT support). Let me pack the daughterbox with whatever GPUs work best with my key software tools, or give me an AMD option that is optimized enough to make CUDA cores irrelevant. Budget challenged buyers still have TB3 for eGPU - some speed penalty vs custom daughterbox, but if Apple puts in some work on the drivers... (rim shot).
Group 3) Everything Bangers
High end. Corner cases, server rooms, media rendering, VR, etc that will always need more resources. Some Enterprise customers, media mastering/serving/distribution, etc. Yes, the bulk of this market is OS agnostic (or Linux lovers), but there are still some Mac-centric educational institutions and custom implementations that don't want to change platforms (whether they should, or not).
I'm also intrigued by the idea that the "brain" tower could be swapped down the road while retaining the daughterbox and it's high speed connection. Most of us on this forum consider swapping internal drives, RAM, etc as easy as making toast - but for a sizable group of customers who might want a high ratio of power to complexity, a single custom "plug" that allows most users to swap out brainbox or daughterbox sounds pretty good. Might support some interesting replace/repair programs to simplify servicing, etc... Think Intel's tick-tock strategy applied to two physical cases.