Another thing a robust tagging system would be good for: duplicate posts and spin-off posts

When you're reposting a meme as a new post, you could add a duplicate post tag, with a link to the previous post

When you post a new meme using an existing format, you could use a spin-off tag with a link to the earliest known use of that meme format, helping keep meme history alive

Also #nostr history in general, since this works for more than memes

And of course everyone would be able to use these tags, not just the original poster