I like the options suggested by @Carl_Haggard - a simple pop-up message when closing out a parent task that still has incomplete subtasks would work. So would a Global setting where Admin could choose whether or not to allow a parent task to be marked complete with incomplete subtasks or whether or not doing so should automatically mark subtasks complete.
Overall, I’d like to see some improvements to subtasks and their relationship to the parent task and project within they live (visibility on boards / timeline, for example).
A little off topic… but it would also be nice for repeating parent tasks with subtasks if the due date for those automatically generated subtasks could be dependent on the due date for the task. Right now if you have task set to repeat with subtasks, the due date is remembered for all subtasks, as is assignee, which is helpful, but it’d be better if it was set to be relative to the new parent task due date. For example, I have a parent task that needs to be repeated every month on the 15th and there are 2-3 subtasks that need to be done prior to then each with variable due dates ‘X’ days before the parent task would be due. I know I can set subtasks to repeat, but they repeat in the current task not the newly created task that is automatically generated when the parent task completes. The due dates for those subtasks should always relative to the parent task when the parent task is set to repeat - basically looking for templates but for tasks.