As of now, overrides are bound to core/directory/game until reconfigured manually. Options to save a custom override cfg (within the specific core config folder by default for better organization) and manually load override files (within that same folder) would make changing settings for different scenarios easier, especially related to overlays and video scaling.
Example of use: Using MGBA or PPSSPP, having the default core override with a handheld overlay and appropriate video scaling, and also the ability to save a custom override with a different overlay and video scaling (example: gba_on_crt.cfg) and load between the default and the custom cfg without having to manually change video and overlay settings each time.
It would also help some new core setups by loading similar configs from other cores within Retroarch, instead of having to manually change the settings for the new core or having to copy and rename the cfg file outside of the program.
Example of use: Loading previously used VBA-M core override cfg, when using mGBA for the first time.