Are there any Dev Environments/Tools for Making Games and Apps Directly on Gameshell?

Pico-8 (commercial, but cheap) and TIC-80 (free!) have development environments built in, but you’d need at least a bluetooth keyboard (and ideally, mouse), and it would be difficult and hard on the eyes to try to just use the Gameshell screen for the display. A better option would be to use a laptop of some sort, run Pico-8 or TIC-80 on that to develop, and then transfer the results to the Gameshell to test. There’s also GB Studio which is free, but again you’d need a laptop or something to run it on: GB Studio - Create Real ROM files

It would be a real challenge to try to make a game development tool using only the d-pad and buttons of the Gameshell. It would be interesting to see if someone tried, but I can’t imagine that it would work that well. Even a sprite editor would be a challenge without a mouse (or touchscreen).