For the 'Project > Open Tasks' menu
Sub-tasks could be automatically placed underneath their parent tasks. When you moved the parent task, the sub-tasks would come with it.
Indentation of sub-tasks would also be useful. If sub-tasks are indented underneath their parent task, it would be easier to tell which sub-tasks belong to which parent task. It would also be easier to find parent tasks, and would alleviate searching through an abundance of sub-tasks when looking for an unrelated parent.