So glad this conversation is picking up momentum.
I think it is significant to note that while #instance-managers can offer a lot of support to each other, for many instances there will be a bigger pool of people that know and support the platform than just an instance-manager. While for sure slack needs a big channel of support for instances, the instance manager circle might not be the same as the Slack channel.
I see the instance-managers circle and a circle of people representing all the instances and helping to make decisions on behalf of their instance within the global community.
An example of such a process is outlined here. It is about supporting instances through the global sysadmin pool to help deploying new releases. That post describes two levels of sysadmin support - basic and full. We need a group that helps to decide which instances receive which level of support based on an agreed set of criteria and in respect of instance contribution to commons (time and money) and the needs of each instance. I think a circle of representatives from all instances would be a great circle to help with those decisions.
It would also be great is a representative from the instance manage circle participated in delivery train meetings, just as we are trying to have a rep from delivery train attend the global hangouts. This will help to ensure that instance needs are represented.
It would also be great to be able to have a specific place to go to ensure that any processes are known by all instances. Eg helping support teams to use the product map that gives the product team insights into which area of the product is causing the most pain to users.
All of these things would be great - but the first steps feel like just bringing people together 
Looking forward to following this progress.