We are kicking off this project in the next few weeks and will provide updates here regarding our status as we move along. Our current plan is to implement Project Settings first followed by Project Permissions once the technical details have been ironed out.
You can also refer to our public roadmap here: https://www.atlassian.com/roadmap/cloud?selectedProduct=bitbucket
Users can only modify repositories' access in repository level although it's grouped under the same project.
It will be best that Bitbucket Cloud can include Project Permission.
This will allow users to manage all the repositories under a project easily.