That depends on when AMD and Nvidia can make them available. The MBP, strictly speaking, is a DGPU niche. It emphasizes TDP over performance, even for any Pascal/Polaris GPU. Historically, this has required special arrangements between Apple and the GPU provider. For example, in the 2010 13" MBP, they obtained a custom 320M from Nvidia. Last year they obtained a previously unknown R7 M370X from AMD.
Such a custom request takes time to accomplish. AMD or NVidia may not agree to devote resources to help Apple with such a custom GPU, and if they do, it's not clear it'll be an immediate priority for them; the R7 M370X after all was a 3 year old core when it came out with the 2015 15" MBP.
Apple chooses GPU designs not to maximize gaming performance, but to ensure they have enough GPU power to sustain the video performance they seek generally. The current 15" in particular is a design compromise. They built it to accommodate a ~35W DGPU, and the IGPU-only versions effectively have a hole where the DGPU would have been. They'd probably want to avoid that compromise and not have to have a DGPU at all.
The 13" dropped the DGPU as soon as the IGPU was powerful enough to drive their graphics requirements. Similarly, the Iris Pro 580 constitutes the first time an IGPU outdoes an existing DGPU. Therefore, it's reasonable to consider that Apple may thus drop the DGPU as a default choice altogether, and charge you extra if you seek it. You may not like it, but there's historical precedent in the same thing happening to the 13" model.