Sub-task type association with parent-task type

It would be useful to create an association of sub-task types with a given parent-task type.

I've create a risk register system, whereby I have an issue type of "Risk" and a sub-task of associated treatments of types "Treatment"

I would like to be able to associate the "Treatment" type with "Risk" so only sub-type issues of that type can be create to "Treatment", but "Sub-task" can still be made for existing issues.

This would prevent the user from having to make a decision about which type of issue should be created.