I think as power consumption only gets worse for GPUs, external GPUs is the solution that makes the most sense...
It would be ideal to have a PCIe Thunderbolt connector that allows you to attach a GPU that maglocks to the back of the screen while in use, and any manufacturer could make their own variations. Thunderbolt can easily send 100w of power. Wouldn't be on desktop scales but it would be enough to make me happy.
In my mind this is ~4 oz. of extra weight, I feel like sufficiently strong hinges and just enough weight in the base would be fine for that little extra though.
(Doesn't make it upgradable of course, but could offer a better choice for people that want a gaming laptop that's also an ultrabook)