Some time ago I proposed a new access control system for Drupal, which would allow the creation of access rules based on various criteria, kind of like the filters in Views.
For example, you would be able to create a rule that would allow view access if the user was logged in, was in a certain role, and the node being accessed belonged to a certain taxonomy term. But you could also create an access rule that only allowed editing if the user was the author of the node being edited, or only allow access to nodes belonging to a certain taxonomy term if the user accessing them belonged to a specific role... Think of Rules for permissions. The possibilities are endless.
I never had the time to study the possibility further and my proposal never really got off the ground but in a group like this, that is dedicated to access control in Drupal, perhaps it can be discussed further.
Any thoughts?