International law in a new society wouldn't exist. Allow me to explain. Let's say the goal of reimagining society is to create something that is more participatory: self managed in line with the values of parecon and parpolity. The hyperlinked writings are worth reading in their entirety to get all the details, but the basic idea is that society's basic political units are primary "nested" councils of people who make the laws; these councils are small enough that everyone can deliberate decisions (think about 40 people or so). These councils federate with each other by sending delegates to higher level councils and society is built from the bottom up. Nothing like a national centralized government exists; decisions are only made on a large scale to the extent that there is a common interest justifying taking authority away from the primary councils. Laws are made only to govern the number of people affected, irrespective of countries or national boundaries. If you don't have nations then you can't have "international" anything, including international law.

Really the question for a new society is one of global law: what would law look like at the largest level of the federation of participatory communities.