Related to the list is a very good feature, but it could be connected only on a level of specific list and not globally or in a folder, which limits its usage. I suppose there is a good technical reason for that; it's a reason why I suggest the third option: make the relationship relatable to the task type. For example, tell that the relation must be just with a task type "Milestone" (or Contact, Idea, whatever...). For me, it would be a huge improvement, because searching for milestones in the structure or all tasks by their names is very ineffective.