Proof of Humanity. There is some work about using Zero Knowledge Proofs as a way to be able to indicate that the owner of a key can also prove ownership of another set of credentials without having to reveal these credentials to third parties. This would allow us to really get rid of bots and sockpuppets.
The ability for users to bring their own cryptographic keys and actor id. This way even if a server goes down people could port their whole account over to a different server.
Multi-protocol federation.
Get rid of downvotes/upvotes and replace it with multi-dimensional scoring/ranking system.
User-defined sorting/ranking. I do not want to completely block people, but I do wish to have a system that could boost/de-emphasize posts by certain people on certain topics, and completely ignore them in others.
Cooperative media storage and distribution that could leverage the storage from clients as well as servers, something based on bittorrent.
Custom widgets that can be attached to a post/community. For example, I’d like to have a play-by-play tracker for basketball/football games.
RDF/Semantic Web descriptors. If people are talking about a TV show, or making a list of PC components that they want to review or anything that can be part of a knowledge graph should be linkable and browsable by a specialized browser.
Collaborative lists/articles/posts. With the item above, it would be trivial to create wikipedia-style posts where a community can build their “common knowledge” and would make it easier for newcomers to get general recommendations and/or a sense of the community values.