I know nothing beyond what’s been reported, but here’s my guess about where Apple’s high-end chips are going:

There’s no M4 Ultra because the next Mac Pro will get an M4 or M5 that supports multiple dies/packages and discrete RAM. To really pile on the CPU/GPU cores, they’ll be able to add more chips. Maybe several Maxs, or a variant with some new interconnect.

Presumably with some RAM packaged with each chip and a lot of magic to deal with the different memory latencies across the different processor and memory chips.

That’s how you build a truly scalable high-performance workstation for AI training as well as video/audio production.