The main issue I see there is the power still has to come from somewhere. All this would likely do is move the pci-e power plugs to the motherboard and cause lots of confusion related to: "this GPU only requires two plugs of power, but the motherboard has three plugs, do I need to plug in all three? My PSU only has two pci-plugs."
It's a step forward. ATX, though convenient and easy to work with for human fingers, could really be simplified for modern purposes. We've just been tacking stuff on for decades.
Its been speced out for a couple a years but the spec always comes out years before we see any motherboards capable of them and years before we have devices capable of them. Are there any x16 pcie 5 gpus out yet?