I think tag aliases are fine, but in my opinion, tags should not have hierarchies. That is just opening the can of ontology worms, and most systems are ill-equipped to deal with ontologies...including ontological systems. Tags are just dumb strings which label data. They are basically KeyValues, where the value is just always equal to True. We don't think of KVs as hierarchical unless they are explicitly a path strin…
That would be a set of values :-)