Today, the only issue types that can be children of a parent issue are Sub-task and Technical Task. Even though I'm free to create custom sub-task types, this is still to limiting. Use case:

I have an issue that requires fixing two bugs, implementing and enhancement, and creating some documentation. I would like to create a parent issue to reflect the main issue and 4 child issues: 2 Bugs, 1 Enhancement, 1 Documentation. Yes, I could create custom sub-task types (Bug sub-task, Enhancement sub-task, Documentation sub-task), but this would end up dirtying the database with too many types.

Further, I might want to show a Workload Report in order to report how many bugs, enhancements, and documentation hours are allocated to members of the team. Displaying hours allocated to an issue type called "Sub-task" tells you nothing. If I added a bunch of new sub-task types, I'd have to mentally add up the types (Bugs + Bug-subtask, Enhancement + Enhancement subtask, etc.).

What I really want is to allow those main types (Bug, Enhancement, Documentation) to be either parents or children.