I like the general idea of having it as a giant gameboy on the go, however I think it would just be too heavy - if it weighs as much as the SP4 with the controller it would go to 1000 gramms weight - which for me personally will likely be too much to hold.
However while I think the active cooling is a neat idea, making it into the tablet is gonna be hard. You can surely point a fan on the hotspot of the tablet however to maybe make it a bit cooler
And the SOC is likey not to be socketable due to the fact that Eve stated they want to use a Core M for better battery-life.
Finally, I think it would be possible to have a GPU inside the controller as you said, but that would have to be a mobile GPU and that might limit the longevity of the device.
If you want to "game on the go" I think just going for a wireless controller and putting the PF on somewhere might be the best way to go - while you might want to keep in mind that the power will be not as high as a dedicated gaming laptop
A modular system on a laptop sounds like a great idea, however as @AntonyTerence mentioned the market is to niché for a dedicated gaming tablet.
You can even replace the GPU on some of the higher-end notebooks, like some Alienware ones. And I've never seen anyone do that, so I don't think the modularity in a laptop would hold a huge market. At least not in the "classica" modularity-way.
If we could make somethink like Project Ara, that might get quite a bunch people interested. HOWEVER we're basically missing the software-support for such a device since that would require a huge amount of customization to have the CPU be interchangeable.
TL;DR: I like the idea, not sure if the market is big enough for such a device.
Just my 2 cents on the topic