In some cases it would be very handy if a sub-task could be assigned to another Jira project than the parent issue type.

example: parent issue is a product requirement for a given product (=Jira project). the requirement is relevant for other products as well - or other products have to do work for this requirement. It would be helpful if these dependencies could be modelled via sub-tasks. if all sub-tasks are done then the parent issue is done as well.