I see it as an evolving process. The basic idea would be that if some group of people wanted to forma family, they could sign some legal contracts to make it official. It could be three lovers. Or it could be a woman, her aunt and her child. Or any group of people that believe they are going to be together long enough to want to go through the legalities. (Let the marriage ceremony or family agreement ceremony be something deparate and up to culture or individual preference.)
Down the road, I could see standard "family agreement" packages. Like a 7 year marriage contract. Or polyfidelity long term contract. Or any other arrangement that is standard enough that lawyers can draw up some standard agreements.
There are always to end the contract early, but the people in it are protected from some of the risk they took. So you would have similar concepts to alimony or child support for breking the contract early.
So while I see polyamory as being relatively fluid in relationship status, many legal arrangements are based on assuming that the partner will be there long term. That allows for one person to quit working to raise a kid. Or allows for some people to pool their money to send one of them to college.
So those who do not feel they can commit to a period of time together, they should not enter into such an agreement.
The common law issue is very interesting. It is designed to protect people who adopt a relationship where marriage rights are to be assumed. However, with polyamory, it is hard to guess what relationship they should be assigned since there are so many options. I am divided on whether common law marriage should go away. So I will pass on speculating on that for now.
Lots of stuff to think on.