Milestones & Sub-Tasks
You should be able to create milestones and then under the milestone, add in the tasks and sub-tasks that need to be done in order to get the milestone done. Each task and sub-tasks should be able to be able to be checked off when its complete and the task(s) should be able to be assigned to multiple people.
When the person logs into their account (admin user), they should be able to see their tasks on the main dashboard via a widget. Each task should be a link to the actual project and task.