As you know, AEM provides 3 OOTB roles: editor, owner and viewer. Our need is to have custom user roles defined in roles selector in folder properties dialog (Permissions tab), say Creator role.
I was thinking about setting a custom implementation over ootb ones, however, the roles list isn't feasible to extend due to com.day.cq.dam.cq-dam-core bundle privacy where all the definitions lie. (com.day.cq.dam.core.impl.team.DefaultRoleProvider as an example).
Thanks for specifiying this. I don't think that this is extensible, but I understand the need for it. You should probably file a feature request using Daycare support system, and ask for a way how this can be extended.