Creating different curator roles has indeed been discussed before, and the current status (4 years old) is summarized in this post and topic:
https://forum.inaturalist.org/t/remove-all-curators-and-start-from-scratch/140/25
As with many other things, it has since taken a back seat to just keeping up with the scaling issues of the site. Although since that time, things did change to require prospective curators to apply directly to staff, instead of being able to be created by other curators.