When I first set up RetroArch it was a pain trying to map my gamepads using Phoenix. Especially with certain cores like Genesis Plus GX, where it’s not clear how the SNES layout maps to Genesis controls until you do trial and error testing. It would be a lot easier to remap everything if I could do it in RGUI, like how you can in MAME’s OSD. Sometimes I forget what keys I’ve set for things like savestate or take screenshot and it would be a lot quicker to pull up RGUI to reference them instead of having to look at my cfg file.
I’d also like to be able to map multiple gamepads to the same inputs like you can in MAME or Mednafen. Right now you can only map the keyboard and one gamepad to player 1 controls for example. And only by editing the cfg file directly, since if you map gamepad controls then try to map keyboard ones afterward in Phoenix, it nulls out your gamepad controls.
It would be kinda cool to be able to map turbo buttons too. Like for NES you’d have a mapping for A, Turbo A, and B, then Turbo B.
It would probably take some work to add this since ideally you’d want core specific layouts in RGUI so you know what you are mapping (A B C X Y Z Start for Genesis, A B Start Select for Game Boy, plus all the directions, for example). It would make RetroArch easier to use though and could be a good feature for 1.0.0.