QMK has various features that aren't currently exposed through the Configurator. Mod-Tap is probably one of the more useful ones, and one of the easier ones to support.

Mod-Tap makes a key act as a particular key when tapped, but act as a modifier when held down.

Since this just uses specially encoded keycodes, it requires no additional firmware support. system76_ectool --access=hid keymap 0 5 0 6129, for instance, already works to set the Launch keymap so Left Ctrl acts as an escape key when tapped.

The main trouble here is coming up with a good UI. Instead of just selecting a keycode, the user would need to select a modifier (ctrl, alt, etc.) and a keycode, with the somewhat weird limitation that not all keycodes work with Mod-Tap (only up to 255).

It may also be useful to allow adjusting TAPPING_TERM, the number of milliseconds the key must be held to count as holding rather than tapping. (QMK even has a mechanism to set this per individual key.) But that would involve firmware changes.