... this could be a nice little desktop for very simple work-loads.
For a few weeks, I had a RPi3B on my workbench, while I was prototyping with it and setting it up (it now does full-time duty as an industrial control data acquisition system, which is a lot of big words for saying it measures and records a few water pressures and tank water levels). During the setup time, I had monitor, mouse and keyboard connected, and for fun I started X on it. Works fine, you can run a web browser, you can edit text in multiple terminal emulator windows. Boring. Just a normal Unix desktop. A little slow at time, but at $35 and 3W power consumption, I can handle that. So if the RPi3B was already a nice little desktop, the model 4 would definitely do it.
BTW, it is time to add SATA port.
With the USB 3.0, is that really necessary? And given the form factor, and the size of a SATA connector, I'm not sure that would work well, without making it physically incompatible with the established ecosystem.
And, as far as I know, the gigabit interface can really do gigabit speeds.
Which brings up a question: For what workload and application would the CPU and other interfaces be able to actually serve the 100 MByte/s that true gigabit can transport? Given the CPU speed and number of bytes, there are only 60 instructions one could execute per byte. That's not very much.
... I was really hoping the next one would have an M.2 slot for an SSD.
That might be feasible, by putting the M.2 "socket" on the bottom side of the board, parallel to the board; sort of in the same fashion that the SD card slot is mounted today. It might barely fit. Given that all the engineering information (circuit diagram, PC board layout) for the Pi is public, I think it should be possible for a vendor (either the Pi foundation or someone else) to try to build such a device, although I'm not 100% sure what the IP situation is.