deskthority

With stuff like the Iris or Ergodox, I feel like people could very easily add an optical sensor in order to turn one of the sides into a mouse. Would this interest people? I know this was done on a board called the Keymouse I believe.

I use an Ergodox and I like both halves being firmly stuck to my desk, I feel that would be either a very annoying mouse with the rubber feet attached (also it's pretty large and clunky), or a very annoying keyboard if it slides smoothly.

To be viable, you must have a good way to grip the mouse or provide friction against your palm. The mouse sensor and centre of gravity should preferably be right in-between those fingers that are used for gripping.

you can use keys to move mouse cursor in QMK (ergodox... etc...)
https://docs.qmk.fm/#/feature_mouse_keys

Well there was the KeyMouse, but it is crazy expensive and I'm definitely more a fan of the QMK mouse keys (or keynav on Linux or Karabiner Elements on macOS) for quickly moving the cursor when necessary. I live in the terminal/browser and use Vimium or similar in my browsers to avoid the need to touch the mouse/trackpad as much as possible.

TMK supports mouse keys, as well.

https://github.com/tmk/tmk_core