This concept can be generalized even more into droning or “pads” which are very popular now in certain types of music like worship music. There are several “Pad” apps for smartphones and tables that allow the guitar player to choose the key for the drone.

Many early primitive instruments included sympathetic or droning strings or pipes which would hold a single note, often the 5th of the scale, or harmonic interval 1-5, while the player played over the drones.

Bagpipes and uilleann pipes included drones. The drone notes on the uilleann pipes can be changed on the fly. Indian music uses the tanpura or a shruti box. The shruti box can be changed on the fly by opening and closing valves covering the reeds.

Many contemporary worship leaders use “pads” which are just synthesizers droning on 1-5 or 1-5-7 etc. fading in and out with lots of ambiance – reverb, delay, modulation.

I think a general purpose droning pedal, especially focusing on the more contemporary use of droning, like maybe automatically sensing the key, would have a large market. Just have a look at how many people are paying for the pad apps in the app stores.