I'd love a fediverse-wide tagging system for various kinds of NSFW (porn, gore, death...), Sports (ideally also by discipline), Anime, Music (again, with genres), Gaming etc. to hand-pick what I absolutely don't want to see.
On some Mastodon clients you can block hashtags, and on Lemmy you can block communities, so we're already pretty close. To do more would require a lot of self-tagging of posts and not everyone will do that. Or it would require analytics and automatic tagging that may not be accurate and would theoretically be a privacy nightmare.
Do it like AO3 and enforce only about a dozen mandatory tags, and allow 'choose not to tag' tag if you don't want to tag, and you can choose to filter out the choose not to tag content..