As an example, KICKR bike has many buttons on the bars. Would be nice to custom map those to functions in the app. For example ERG +/- when trying to train HR zones, or skip a section of a workout etc. Basically any of the companion app buttons.
If you wanted to make this super easy for people, you could also enable MIDI protocol so users could use any of the thousands of cheap midi controllers out there. Super easy to implement.