Unless I'm misunderstanding your meaning, you can already do this. When reblogging a post, you can just add tags without adding any other content, and it adds those tags to your share of the post. (It does add it onto the search pages for those tags, which is really weird behavior to me, but that's kind of a separate issue and only important if you're adding categorization tags, rather than just adding Tumblr-style commentary.)
@jgs Oh really? That's great! I haven't been able to post yet and I didn't see any examples of that yet so I had no idea. Thanks so much!
onion
Myself and every other content creator I've talked with misses reading tags. People are much more willing to say stuff when they feel like they're not speaking directly to OP (like a reply) and it was a really fun experience that I'd love to see come back. Like tumblr, your tags you added would only show up on your specific reblog. Cohost looks to be halfway there already with tumblr-style tags on the original post, and I hope they consider it.