Are there any plans to introduce roles and capabilities for authors like we have in WordPress (Admin, Editor, Author, Contributor)?
Publii is an on-premises, single-install website editor. It isn’t a multi-user, software-as-a-service, cloud-based solution where people can log in, have user accounts, roles and capabilities.
While it is absolutely possible to share the management and administration of a Publii site by using a shared folder as the active “site” directory, and allowing people who have access to the shared folder to modify content, I do not believe there’s any way to control access and publishing rights in the way you describe, like you might a cloud-based or server-side solution, like WordPress.