We see the need for decentralised anti-abuse and reputation systems as the single biggest existential threat to the growth of Matrix.org (which otherwise risks being almost worse than SMTP). It's a problem that the whole decentralised web community has in common, and a solution could benefit everyone equally. We've experimented with data entries in Stellar ledgers for this already, but it needs a lot more thought & work. We tried to issue a call to arms at FOSDEM this year to try to get people thinking more about it, with a bunch of possible ideas very similar to this post (see slides 27 to 32 of https://matrix.org/blog/wp-content/uploads/2017/02/2017-02-0...) and it would also be absolutely awesome if the W3C social groups could help drive a solution.