I should be able to filter a list based on custom fields – specifically Collaborators on a task – so that, for example, I can set up My List to include tasks on which I’m either the Assignee or a Collaborator.
Similarly, I should be able to run Rules that include conditional statements about Collaborators (such as, “Notify all collaborators of changes to any of their tasks.”
And, I should be able to run this so that it doesn’t duplicate notifications to someone who is the assignee - or choose not to have assignees added as collaborators on tasks as well.