Old accordions and concertinas are often out of tune. It turns out that when I bought my accordion on ebay I was actually pretty lucky to get one as close to in tune as it is. Harmonicas are pretty cheap, much cheaper than tuning anything with reeds, so what if you designed an accordion or concertina to have a replacable harmonica inside? Pick a standard and common harmonica and then either take the reed plate out or use it as is. If you made it easy to swap them out then players could suddenly have access to many keys on cheap squeezeboxes.

(I know that accordions, harmonicas, and concertinas all use different reeds, but at the low end people care less.)