The rumors about supposedly credible leak about about Apple's "Legos project" Mac Pro design that was tweaking it for Thunderbolt 4. ..... When there is no Thunderbolt 4
where just how believable is was that ?
https://www.macrumors.com/2019/03/04/usb4-converges-usb-and-thunderbolt-3/
Now that Thunderbolt will be driven by an even larger committee any major upgrade in specs will probably move slower ( order of magnitude more cooks in the kitchens probably won't make things at quickly has they have). USB 4.2 , 4.5 (or 5) might get an iteration.
Also kind of chuckle of the bucket full of "Thunderbolt is dead' posts on these future Mac Pro threads over last 3- 4 years. .... About as "dead" as NeXT was a couple of years after the Apple take over (that was just about as much as a reverse take over). Thunderbolt on the next Mac Pro is highly likely at this point if it is better coupling with future USB 4 devices. (and likely would have USB 4 in some distance future iteration if gets another upgrade at this one. )
Is Apple going to 'fork' from Thunderbolt 3 to do something off standards and lone wolf with their own chipset development? They could but it probably won't buy them much other than more drama later on. I don't think that is path they are on.