With all the search and processing power Google has, I can't see why they haven't done something like this.
When I'm viewing a conversation involving just me and one other person, I want a list of recent conversations readily available in the gui, like in the mockup below. (This gets trickier when more people are involved, but for one-on-one conversations, it could be a simple search. Nothing fancy.)
The almighty Google should also be able to give me a list of related conversations. If I'm juggling emails back and forth with John about project reports, why not show me the conversation I had with Jane on the same topic last week?
Go on. Build it.
Oh, and cake. I want cake.