Ive gotten to a point where I think this concept has finally materialized.
A Gameboy pocket, with a 3.5in screen, powered by a Raspberry Pi 3A+
This was not an easy or straightforward project by any means. The front shell needed to be CNC'd to accept a 3.5in screen. And the screen needed to be disassembled so that it would sit flush in the front shell. So the metal housing that normally surrounds the screen was pulled off and then the LCD and the LED backlight were then separated. I developed a master board with help from files provided by Mattc and Eugene (32teeth). The masterboard has a 18bit DPI adapter and the common ground button setup.
Features include: adjustable brightness in the contrast port, USB C charging, and this is a minor thing, but the LED status from the Pi shines through the power switch. just thought that was a cool extra that just happened.
Ive always like the Pocket's design. Nintendo has a history of great product design, but as far as handhelds go, the pocket is my favorite. but it doesn't seem to be used often due to its extremely small size.
Feel free to ask questions. I enjoy sharing my insights and I have to thank Rodocop and Tinkerboy for their ability in helping me out. Those two almost single-handedly answered all of my questions, but of course others from this forum have helped immensely.
If you want to see some vids of this thing, be sure to check out my instagram; I did a sort of walkaround after I completed it.