I would almost predict Nvidia as the next card simply because AMD was the last pick and Apple seems to jump back and forth between the two:
It is hard to tell until Apple releases but was the 2013 Mac Pro done inside the scope of 2012 designs or 2013 designs. In 2012 designs (done somewhat in 2011), Nvidia "won". The last Mac Pro design win for a GPU card, AMD in 2010. Depends which scope looking at Mac or just Mac Pro.
Baseline MP builds:
2006-2007 GeForce 7300 GT
2008 Radeon HD 2600 XT
2009 GeForce GT 120
While the entry level card, Apple did have a line up in these three that didn't include of of the "other".
This is only point where they went totally exclusionary. A Quadro card showed up in 2011 but never part of the BTO mix. The exclusionary thing is somewhat of an outlier. Whether it is now the "norm" is hard to tell because they haven't done anything since then. It could have simply been an artifact of putting the Mac Pro R&D efforts into cryogenic storage. By all appearances that's effectively what they did.
CUDA support certainly isn't a factor in that list.
I know but it is almost always lurking under the covers in contexts were folks are highly focused on video processing with Adobe products.
The only thing I get out of their history is that Apple doesn't want to get locked into a single vendor. Apple is probably keeping both in the game in order to have them bid against each other for volume orders.
I also don't think Apple is out to overly support CUDA and Nvidia's proprietary solutions either. ( one of the initiators of OpenCL for one ).
However, Apple is rather Scrooge McDuck. If they can squeeze more volume out of using Nvidia and it fits the design parameters they are generally looking for "power efficient" GPUs then there is a chance Nvidia may go Nvidia again across the whole line up that are getting changes. AMD GCN trended a bit toward boosting power to gain performance on this iteration.
I think Apple does want to keep both in the game but to play "every other" the two can't get complacent and think Apple has to pick them because it is their turn. If Apple is pushing certain parameters and they don't deliver they give it to them just because "not the other guy". I'm strongly suspect Apple gives AMD a shot at CPUs each round too, but they never deliver on a competitive basis so don't get the nod. But they do wan to be able to go to Intel and said ... if you screw up we're walking.
To keep them both in the game I suspect Apple would split between the embedded GPU and the one that fits in the PCI-e card slot. For example,
AMD 8750M 2GB embedded and a Nvidia card ( 660) as the entry level default.
or
Nvidia 650M 2GB embedded and a AMD card ( 7870 ) as the entry level default
The 8650M because nominally designed for x8 PCI-e v3.0 connection and generally keeping AMD in the game. The 650M (or slight bump update later this year) because used across multiple products which would lead to cheaper unit costs on volume.
That would put Apple back in the mode of not being exclusionary. They probably would sell at different volumes (especially if lower cost BTO config were can get only embedded GPU) but it keeps both players in the game on each round.
