What do you think The Empty will be used for later on in the show? Since it's just too interesting a concept to leave, and our Cas is back, what would they use it for?
I personally see it as having served it’s purpose, emptiness is often linked to depression so the literal/metaphorical aspect was excellent and just what I expected for Cas. So I don’t think it has to be revisited at all, but they could use it to bring back other characters who are gone or something.
I really don’t have a narrative speculation to make based on current canon though like I have for other things because as I say it served it’s purpose and I don’t know how this would be particularly relevant to others other than just as a plot point, which is a possibility too though but less easy to speculate about as it’s just a useful tool rather than a meta/metaphor based point… so I’m open :)