If you want second homes to be used productively, just allow folks to list them on AirBnb for planned short-term rental. Long-term rentals are a total non-starter politically for second homes, since you obviously can't ethically throw out someone who regards that rental space as their home. But short-term is actually fine.

As for larger homes, people should be allowed to live in there as larger, extended family groups - a common pattern in non-Anglo cultures. Ban "single family" restrictions since they amount to unconscionable discrimination against such reasonable living arrangements.