This feature would be extremely useful for me, and maybe for other users:

When a Task is in the Waiting list, maybe it is waiting for other Tasks to be completed. For example, "Build new website" is waiting for "Discuss new website with marketing" and "Decide on platform for new website".

When you are editing a Task in the Waiting list, (or when you move a Task to the Waiting list) it would be good if you could define a list of one or more Tasks it is waiting for. Maybe you could pick from a list of other Tasks in the same Project. When all tasks on this "Waiting for..." list are Completed or Deleted, then the Task moves from the Waiting list to the "Inbox" (or maybe to "Next").