There's a bunch of questions about adding chat onebox support for this, that, and the other.
- ThinkGeek onebox support for chat
- Add support for secure Wikipedia pages to Onebox
- Gist support for chat 'onebox'
And then there's a super-question about the same thing, with various proposals as individual answers.
So if, for example, I want Wiktionary to be included in chat oneboxes, where should I post that? (What I've found so far is one answer about MediaWiki on the super-onebox question, with a comment about Wiktionary. Is upvoting that (which I have done) enough?)