[beating my own drum:] Indeed, I have a working (rough draft of) realtime collaborative editing at https://www.mediawiki.org/wiki/Extension:TogetherJS.
It is quite an interesting UX when you allow realtime "writing at" each other in this form. Old-timers will remember "ytalk" chats as a very different sort of discussion, with its own conventions.
At wikimania Erik and I will be presenting two sequential talks (or maybe one combined talk, TBD) on real-time collaborative mediawiki editing, and what it would look and feel like and how that would affect community interactions. (As a brief plug: I would like to make WP feel more like an active community of *people* and less like a sterile collection of pages authored by some invisible cabal; from some previous threads about the "last edit" banner on mobile I'm sure there are some who prefer the anonymous font of knowledge look. But ultimately for real-time collaboration to be useful, a prospective editor needs to find someone to collaborate with...)
Among the interesting issues is how to handle fine-grained attribution of content/changes in a real-time collaboration. This might also influence how we feel about fine-grained attribution of comments in a larger talk page. I look forward to hearing ideas and discussion. --scott
[weighing in on the nesting/quoting bikeshed: the structured quoting which Simple Machines Forum (SMF) provides is a nice compromise: it preserves the exact origin of the quoted material, for easy backreference, but it also allows flexible editing of the quoted content and for combining multiple quotations into a single response.]