If they stick with UMA, gpu performance is going to be severely limited, unless they can make the unified memory GDDR6 or something.
[1] https://www.anandtech.com/show/15885/hpc-systems-special-off...
It's more likely that the future Mac Pro will use Lifuka which everyone in this thread is ignoring.
It would be interesting if Apple did make a discrete GPU, that would means they are making a separate GPU die for their Mac Pro and iMac with volume estimated to be no more than 1M unit per year. It makes sense from a technical perspective, it surely doesn't from a cost perspective. Although Apple could happily charge $1000 just for the GPU to make up for ROI.
The advantage of GPU having it's own memory is you can use super high throughput ram that the GPU needs, while using higher capacity/lower energy/cheaper ram for the cpu.
The downside of using separate ram is that you have to shuttle data back and forth from one ram to the other quite a bit, as well you get less flexibility in how your total memory is used.