The one killer feature missing from email, and which should be doable with an standard extension, is to retroactively include someone in a conversation, forward

The one killer feature missing from email, and which should be doable with an standard extension, is to retroactively include someone in a conversation, forwarding the messages you have a notifying other participants of the change. I note that newsgroup are somewhat that feature, but they live in a different space than email, with no integration. I really wish companies would develop and push such a standard instead of reinventing 80% of the wheel, tack on their shiny new proprietary hubcap and try to displace email. We have people pushing slack here, and it's annoying to have yet another independant channel. It doesn't properly cross polineate with email.