I think that the market doesn't really exist for such things, as cool as they are. Such solutions have been around for a while, and they've never taken off.
I would personally like the option, but don't think it'll happen.
External GPU has been around for some while, but they lack a standard port for connection, only Thunderbolt-3-over-USB-C offers the necessary bandwidth and compatibility for wide-spread usage.
Id much rather have a virtual pc in the cloud with GPU abilities I can access from anywhere rather than lug an external GPU around.
You would need a super-fast and super-stable connection to play games with a virtual PC in the cloud.
I don't know if there are any licensing fees, probably not that much, but consider the fact that by allowing owners to beef up the gpu of the surface pro, why would they then go out and pay a significant premium buying the surface book. Again, I disagree with the argument, and personally think that microsoft should have just ditched the surface book and tried to develop that concept into the surface pro, like the power cover etc.
Surface Book and Surface Pro + External GPU will serve very different segments, and they can and will co-exist quite nicely.
Just remember that the surface is not aimed to compete against any gaming machines. It is aimed at a professional useage case, not a gaming one. I think that the percentage of surface users who would want such an option would be too small to make it worthwhile. It would also impact the surface book which uses the dgpu option as a big differentiater between it and the surface. I personally just can't see it happening.
Outside of gaming, GPU still has a huge impact on "professional usages" - Photoshop, Premiere and After Effects will benefit greatly from a powerful GPU, even against the under-power discrete GPU of the Surface Book. Maybe you don't see it, but I can see the appeal of a Surface Pro that I can take anywhere with me, and when I settle down at work or at home, I can attach a external GPU and a 5K or two 4K monitors to it, either to render some images or videos, or play some AAA games.
Anyway, I'm convinced the SP5 and SB2 will both have USB-C ports, and will both have Thunderbolt-3-over-USB-C capabilities. The question is how many USB-C ports will be built in. Razor Core and other external GPU enclosures with Thunderbolt 3 will definitely be a hot accessory for them.