Make which work items can be created as child items configurable.

Edit Subject

I would like able to configure which work items can be created when clicking the "+" sign on a parent work item (like a user story). It would be nice if I could configure this separately for the planning board and the task board. For instance, on the task board I would configure it so that only User Stories could have child Tasks and Impediments (but not Stories), whereas on the planning board I would allow child Stories to be created.