I'm guessing not that many people have a mouse and keyboard connected to their Mister.
Perhaps those with a very minimal setup would still want to use computer cores?
Keyboard emulation on a joystick is perhaps a bit tougher (on-screen keyboard will probably take a lot of effort), but mouse emulation sounds easier...
I'm proposing a button that swaps the joystick input for mouse emulation, so joystick right moves mouse right, button 1 is left click, and so on.
What do you think?
(Btw, which is the "main" Atari ST core in Github? I wanted to post this suggestion there, but seems there's more than one core, or at least more than one repository?)