The M-6 was popular for a reason.
If any changes are made I would vote for:
#1. a plastic forend, could a scope mount be integrated?
#2. perhaps .22 hornet but its tough to beat plain .22 cal.

IMO stay with .410
Concerning the bird shot the .410 will hit just as hard as a 12ga, the only difference it the pattern density.

Why try to reinvent the wheel !