This is a short Lua script for mpv that allows cycling through existing video, audio and subtitle tracks, skipping the "none" option.

Save cycle-through-existing.lua in ~/.config/.mpv/scripts/ (Linux and macOS) or %AppData%\mpv\scripts\ (Windows). Edit your input.conf file to include the shortcuts for script-binding cycle_{video,audio,sub,secondary_sub}_{up,down}. Example:

_     script-binding cycle_video_up
SHARP script-binding cycle_audio_up
j     script-binding cycle_sub_up
J     script-binding cycle_sub_down