@snacks @lain PCIE extender is a hack solution but considering risers are already a thing and we're trying to make an atrocity work without catching fire, it's a somewhat reasonable solution. The box could just be attached to the side of the main case so the distance between the motherboard and the GPU box would be about the same as a regular riser.
External PSU but the GPU still inside would still be pretty bad for a simple reason: you'd create an oven. 300w GPUs already generate enough heat to raise the temperature of the other components. Separate box allows for a huge cooling unit that won't have any "how do I fit it in the computer" issues with engineered airflow. It will also suck clean air and not interfere with the cooling of the other parts.
We used peripheral boxes for cd burning units and a bunch of other things in the 90s, it's nothing new, the difference is that back then everything was a light beige rectangle and no one cared, that was already futuristic and cool enough. More boxes on your desk: more cool