Allowing users to define their own censors would be huge. While I agree with the posters that have said that sweeping lists of words to block won't do anyone any good, allowing users to filter or censor strings that they themselves define would be an excellent addition.

Imagine an interface where you can add a string to a list. You can determine whether it's case-sensitive, whether non-breaking spaces or space characters should be considered, whether to simply censor the string with asterisks, whether to hide the message it contains altogether, and whether or not to blacklist the person who says it (obviously, the capacity limit on the blacklist would need to be greatly expanded). It would be a small project that would add a bit of customization that would allow users to make the game a safe place for themselves.

Say for instance a user has been sexually assaulted, so any mention of sexual violence, or even of sex in general, triggers severe anxiety in them. With this system, they could then add specific words and phrases to their own filters, which would shield them from discussion that would otherwise cause them upset. It would enable them to enjoy the game more and be socially active, at no cost to anyone else.

This would also help users be able to combat RMT spam on their own, and potentially even export their chat filters for others to use (and SE could even conceivably use the crowd-sourced filters as a default option in the game).