It could be awesome to create collection of tasks, like epics or actually any grouping method with progress indication. Subtasks are not valid tool for this they are too obscure on boards. Main goal of this feature is to create some container with progress indication (done, in progress) that can be used as higher picture overview for big planning.
I don't think that deep hierarchy is needed here, just one level down. Basically group of tasks is task with collection of other tasks and calculated progress of that collection. Since collection is a task itself you can create collection of collections and treat it the same way. Again only one level down is needed.