Other than subtasks, I think priorities and/or color changes for flagging would be a welcome addition. To keep it simple, a color selector for flagging with it being sorted under the "Flagged" menu would be very beneficial. If subtasks and priorities (or some form of) were added, I would completely switch to Evernote for tasking. Those features alone add immense value to the task section, in my opinion.
+1 for subtasks. With checklists having indentation, I hope this is easily an upcoming feature for Tasks. This is the only feature keeping me from cancelling my task manager subscription entirely. Using notes as parent tasks is a great workaround but is exhaustive when you consistently use subtasks for smaller projects.