Every email between Gmail users should autoconvert to a Google Doc wherein replies are suggested edits or sidebar annotations/comments.

If you want to add someone to a "thread" you just share the doc with them.

Would make it easier to join an ongoing conversation without having to consume each incremental diff in sequence like most email threads.

Also helps prevents isolated conversational branches from spawning.

It's not quite "just use a Google Doc" for everything, as you still want it to be more of a conversation...