Why not replace the DB9 connector with a simple 9-pin linear connector and adopt the DB9 MEGADRIVE/GENESIS standard?
This standard is now adopted by some manufacturers, the most famous being 8BitDo. It allows for more buttons while maintaining the 9 pins, and it's easy to build an adapter for C64, Amiga, MSX, etc.
Why not replace the DB9 connector with a simple 9-pin linear connector and adopt the DB9 MEGADRIVE/GENESIS standard?
This standard is now adopted by some manufacturers, the most famous being 8BitDo. It allows for more buttons while maintaining the 9 pins, and it's easy to build an adapter for C64, Amiga, MSX, etc.