Flagging my interest here, too!
I am an Asana Solution Partner and many of our customers (~8/10) ask me for a possibility to somehow prevent parent tasks to be completed before subtasks are - or at least be shown a warning pop-up. Many of them use subtasks as “Check lists” - this mentality seems to be a popular usecase for subtasks.
Alternatively, since it does not seem to be an issue of relevance in Asana, we would greatly love the opportunity to alter this via Asanas API! We’d be happy to think about a custom solution, but as of now the API is not allowing to develop a custom solution for the completion of tasks. Correct me please, if somebody knows more than I do here.
Also: Subtasks should be autoadded to the same project as the parent task. After all, I feel like this only makes logical sense.
All the best,
Lena