Most insightful and original bit is at the end: I don't know what Google has planned for Wave or Gmail, but if I were them I would continue improving Wave, and

Most insightful and original bit is at the end: I don't know what Google has planned for Wave or Gmail, but if I were them I would continue improving Wave, and then once it's ready for the whole world to use, integrate it into Gmail. Moving Wave into Gmail would give it a huge userbase, and partially address the "email is universal" problem. They could use MIME multi-part to send both a non-Wave, HTML version of the message, and the Wave version. Wave-enabled mail readers would display the live Wave, while older mailers would show the static version along with a link to the live Wave.