Can we have a new metric "Monthly returning visitors" that take into account visitors with 2 or more visits in the same month ?
Is this a feature you are looking or proposing? or is this a question?
If a question: you can use a cookie to store the visit number (Which should expire after a month) and read the same cookie in all the visits. If the visit number is >=2, you can have a event-triggered with event serialization helps you get the monthly returning visitors.
Hope this helps.