I bought a 37D GearMotor with a 6 mm Universal mounting hub but wanted to connect to a 5 mm shaft (or smaller). This seems to be problem because the 6mm hub has 6 holes that don’t line up with all of the other 4 hole hubs. If you offered a universal monuting hub adapter that will connnect the 6 hole hub to all of your other 4 hole hub connections, it would make all compatible. It could be an aluminum adaptor with drilled and tapped holes, or the adaptor could be ABS plastic.
The other solution would be to offer the 6 hole hub in other sizes (3, 4, 5 mm) although the demand may not be enough to offer.