What's great about all of these decidedly "high-end" handhelds coming out is the requirements to reconcile battery/heat problems that are posed in tight spaces.
Honestly looking at the Win2, it is JAM PACKED. To fit a 10000mAh battery, heatsink/fan, 2242 SSD, 6in screen, complete gaming controls and keyboard inside of 162x99x25mm.
Exploring different options in the future is going to be a necessity. They might have to figure out how to have SATA wiring going through the hinge and have 2280 or 22110 drives behind the screen. This way the screen component isn't too heavy and we can scale the heatsink+fan to accomodate for a 15w part.
Having said that, we would need 20mAh battery to have the same battery life with a 15w part. A 2x increase in battery size will be noticeable somewhere. And that's a big challenge.
I don't know how much bigger a device could get. Right now the Win2 is about as THICC as I want, to where I can still find it pocket-able. But almost any dimension we increase, it would really be on the fringe case of "pocket-able"
So, the future will be interesting. Provided that SmachZ does come out and makes a product that is nice to use, then I think GPD will really have to consider increasing the power envelope. A LOT can happen in two years. I hope GPD waits until 2020 when AMD is supposed to launch at 7nm. At least by then we'll be looking at 2x PS4 levels of gaming perf inside of 15watts. (I will really try to push for HBM3)