There's no indication because there's no 5800/5900 release date (yet) from AMD, but should be released within 2020 otherwise Nvidia would catch up and make AMD look bad (again). There's no reason Apple doesn't replace Vega II with Big Navi as a) Vega II (aka Radeon VII) doesn't support DSC and b) it's old arch and power inefficient. It's gonna take even longer for Apple to release their "X" versions than AMD does, but doesn't mean they won't come.
"Big Navi" isn't the only option.
https://www.notebookcheck.net/Leake...n-RTX-2080-Ti-crushed-by-Ampere.453533.0.html
https://www.techradar.com/news/amds...er-efficient-but-probably-wont-trouble-nvidia
The Vega II MPX modules are aimed at least as much as computational horsepower as they are aimed at "fast frame rates" and/or "higher resolution screen dimensions". Arcturus is the natural follow on to the "Vega 20" architecture implementation in the Vega II. The same way the Vega 20's MI50/MI60 ---> Vega II Single/Duo , there is a pretty decent chance that t Arcturus powered MI100 ---> new top end MPX modules ( single and dual ).
Apple will have a Navi solution in the W5700X. If there is any Navi solution coming to the Mac Pro it is more likely a "small" Navi to replace the somewhat maligned 580X module.
https://www.amd.com/en/products/professional-graphics/radeon-pro-w5500
From the rumors so far there are some substantive problems with "Big Navi" in the context in which Apple uses them. First, TDP. It is probably a stretch to get to two "Big Navi" GPUs onto a single MPX module. Arcturus may have picked up the power consumption optimizations that AMD put into the mobile Ryzen 4000 Vega implementations. AMD put some substantive 7nm optimization work into Vega for those
"... If you notice, the frequencies are super high – AMD said that this enhanced version of Vega, architected for 7nm, actually responded really well in terms of frequency and power, and so they were able to boost the clocks up a lot. So despite the drop from 11 CUs in the silicon design down to 8 CUs, the 1750 MHz frequency really piles on the performance. AMD is promoting +28% GPU performance in 3D Mark Time Spy against Intel’s Core i7-1065G7, as well as a single threaded performance lead over both Ice Lake and Comet Lake. ..."
https://www.anandtech.com/show/1532...-apus-7nm-8core-on-both-15w-and-45w-coming-q1
If they extended a large fraction of those "rework" optimizations to the Vega 20 baseline that would be a straightforward path to Arcturus. ( much , much bigger GPU so different yields (probably significantly lower) but the Vega II $2,800 & $5,800 price tag probably allow for that. ) . If AMD did a minor upgrade to the Display generator engine(s) to include DSC to the DispalyPort 1.4 those two upgrades would probably be enough for Apple.
I wouldn't bet on "Big Navi" bring a price drop for Apple's high end GPU MPX modules. ( And Radeon VII price has nothing to do with Vega II pricing. )
AMD has may/may not be doing a 5500/5700 refresh late in 2020. But those aren't too likely when Apple hasn't even gotten the W5700 yet ( or even announced something in the 5500 range on the current cycle).
There is a pretty good chance that Apple's W5700X is price pretty high ( $800-1200 range). Something "Big Navi" is will start to run into the zone of the Vega II with perhaps not top end computational horsepower.
Navi computational "advantages" somewhat appear to be mainly grounded in better clock speeds, not particularly better raw compute. If even the clock speed playing field then Arcturus has a pretty decent chance of being better at that. ( gaming graphics no.... grinding out large computations in most cases probably yes ... ). [ And Arcturus may use die packaging to get around any CU cap limit they may have . So bigger CU count wouldn't be exclusive to Navi either. ]
And there's already a PC variant (W5700) available. The "X" version is Apple's custom version just like those Vega II (custom Radeon VII). And MPX modules never were available for purchase anyways since MP7,1 released.
Apple's W5700X is a minor leap because it has 16GB of VRAM and not 8. The Vega II is more so a derivative of the MI50 MI60 than the Radeon VII .
But if want to point at the W5700 the W5500 was AMD's next move. If Apple is just shadowing that by 3-4 months .... that would be their next move.
There may or may not be a Pro W series "Big Navi" in 2020. Depending upon Big navi demand and how far the release slides into 2020 , the W variant may slide into 2021. That may help because there is probably a limited appetite for Apple do do a more than three MPX variants. And the W5700X may run long enough to simply just substitute it later with something in "next iteration" class in the mid-range class.