Thanks for your responses. My technical experience is limited, but I think with tutorials and forums I can get things set up and running. It sounds like the Teensy is the better solution and I did buy a soldering iron.
@gadgetoid There are a lot of people modding the gameboy DMG, psp, vita, wii u, gamegear and other handhelds to use raspberry pi and raspberry pi zero. There are a number of people making all-in-one boards for the gameboy form factor, but the designs are pretty constrained due to the gameboy case.
Do you think there could be a big enough market for a picade pcb designed to be 1) enclosure agnostic 2) capable of having teensy like number of IO inputs, 3) the following all-in-one features (amp for speakers, safe shutdown, battery boost, simultaneous charging/discharging of battery, a port for a volume potentiometer, port for a power button).
Users could buy a raspberry pi or a similar board, a mythical Picade Portable all-in-one PCB, stuff them both into an enclosure with a battery and after soldering up the buttons, switches, and potentiometer be set.
Like people could retrofit almost any old handheld they have laying around and have the possibility of adding more buttons and analog sticks. Other people could use hobbyist designs on 3d file sharing sites like Thingiverse and have them 3d printed.
Maybe it's too niche? But If this did exist I bet a small community of hardware modders would grow around it. Maybe there could be monthly design contests. On the sudomod forums there are people who are already on their 3rd, 4th, builds.