My thoughts are Apple needs to better differentiate the Mac Pro from the Mac Studio. Internal slots are a good start but not supporting GPU expansion with them limits the user base that benefits from them.
An M4 Ultra isn't a sufficient differentiator from an M4 Studio Ultra (assuming there will be one) as it would be no different than the situation we have today with the M2 Ultra. Therefore a more powerful AS chip would be ideal, perhaps an Extreme chip as people like to speculate.
Perhaps adding expandable memory? It's kind of weird to know the 2019 Mac Pro can accommodate up to 1.5GB of RAM but the AS Mac Pro is limited to 192GB.
I guess, in summary, for me (if I were in the market for an AS Mac Pro, which I am not) it would come down to better differentiation.