It's not exactly the same as "inter-discussion replying", but ...
Sometimes I think it would be nice to be able to quote something from another discussion. Ideally, a user should be able to click on the quote to open the relevant discussion in a different tab. Would this sort of thing be doable?
At the moment it's a two-step process: post a link and copy a quote. Some way to streamline the process would be nice (though not really a huge priority).