Stereo/mono toggle & left/right speaker balance
Mono output is useful for unilateral hearing loss, but the number of games that provide it is low.
This is something that should ideally be set at platform level, allowing the audio from all channels to be played equally through all speakers. Speaker balance is also useful, for partial unilateral hearing loss. Both settings are standard functionality on mobile device operating systems.
Stereo/mono toggle and left/right balance on iOS
Next: Voice commands