I play the hurdy-gurdy, and a guitarist friend has joined me over the past months to play acoustic music together. My instrument is in the key of C, and we transpose whatever we want to learn into C (major, natural minor, harmonic minor, mixolydian) so the song resolves nicely onto the drone note.
This has led to some difficulty because the guitarist is unable to quickly set his fingers into position for some of the chords we would need. In particular, the major chords B♭, E♭, and A♭; the diminished triads B♮ and D♮; and the minor chord C♮ are causing problems in the standard tuning. Most of what we play hasn't called for seventh chords, so I don't know how difficult those are.
Is there an alternate tuning that would enable him to more easily play the chords used in these keys? Since the drones restrict us to a particular set of keys, it wouldn't be much of an inconvenience if it made other keys more difficult; you're allowed to optimise specifically for the C family.