I'm no expert ~ I carried an "A" harp around for years with the intention of learning to play blues in E, but never quite learned. My harmonica mostly served as nothing more than a pitch-pipe, plus I never hesitated to loan it to anyone able to play along competantly with my guitar blues in E.

One thing that I occasionally wonder ~ besides "straight" and "cross" harp, does anyone ever play minor-key tunes using a third different key-relationship?

I'm thinking, for example, about using a "C" diatonic harp to play in A-minor, a "G" harp for E-minor, etc. Such an arrangement would provide access to the appropriate notes for a complete minor-key scale, but I've never heard any discussion of such an approach, not like the usual talk about the blues/cross-harp key relationships.