Subtasks with relative due dates
It would be nice if subtasks could have relative due dates (number of days before the due date of the main task). When a main task (with subtasks) is then copied/cloned, the (relative) due dates of subtasks would not have to be manually changed.
I think relative due dates would be easy to implement, for example:
A sub-task with the due date "main -5d" would mean: the sub-task has the task due date minus 5 days.
I used "main" above to be general.