I'd love to have a temporal relevance filter on Twitter. "Show tweets about current affairs", "Show tweets that will be relevant for several years", "Show evergreen tweets", etc.
Generally speaking, there's a lot that could be done to enable users to customize their feed.
As someone who very much enjoys podcasts I continue to be frustrated that so much information is locked up in opaque audio files. How do we make all of this information accessible, searchable, navigable, linkable, upvotable, etc? Great opportunity if someone does this right, imo.