I don't know whether this is a new idea.
We need task folder feature to integrate our other task management systems with Workfront.
1) user can create any number of task folders
2) user can add arbitrary tasks to his folders from any project he has permission to view or above.
3) folder has link to task not the task itself. so same task can be in many different folders without any problem.
4) UI shows task list in the folder just like task list in project
5) user can remove task link from folder
6) user can change order of task links in folder
7) when user open a task link in the folder system shows task detail the same way as it shows task detail page in project.
You can think of a Task folder as a dynamic collection of Tasks view from many different projects.
There should be CRUD API that manages folder and task links and event subscription API that tells event of task getting into or going out of task folder.
