(Disclosure: I work for Google, but not for the Gmail team.)
Google Wave was a high-profile failure, but the core idea — that you can edit a message after you’ve sent it — was great. How we can take this great idea from Wave, and apply it in today’s world, in Gmail? One of the reasons Wave failed was that people didn’t want to check yet another inbox. Doing it in Gmail fixes that problem.
The solution is to have Gmail support editable mails. If the sender and all recipients of an email all use Gmail, the sender should be allowed to edit it, and it updates in recipients’ inboxes . This helps in many cases: correcting mistakes, whether typos or more substantial mistakes, explaining the same thing in a clearer way, changing a recommendation based on new information, adding information you forgot to include, and so on.
If you want to retract a mail you sent, rather than sending another mail saying, “Please ignore the above mail”, which people read after wasting time reading the original mail, you can replace the body of the original mail with “Please ignore this mail”.
Email discussions with multiple people currently have the problem that if you make a mistake, or happen to say something that’s interpreted in a different way from what you intended, one person can point that out to you, and you can send out a clarification, but everyone else ends up first reading the original mail, getting confused, and then getting clarified. Once the first reader points it out, and you correct it, other readers should read the corrected version.
And the person who pointed out the mistake can then replace their correction email with “Please ignore this mail”, preventing others from wasting time reading a correction that’s no longer relevant.
In addition to editing the text, the sender should be able add, remove or modify attachments and inline images. Imagine mailing a Word document, but forgetting to include some relevant information. You will be able to edit it and replace the original attachment with the new one. That way, you won’t have many versions of the attachment floating around, causing confusion. Conceptually, editing an attachment is no different from editing the text in the email.
You won’t be allowed to edit the subject, since that messes up threading, causing confusing behaviour like a mail migrating out of a thread to its own separate thread, or to another thread. And back, if you revert the edit. This will be confusing, so prevent editing the subject.
Mails can also be edited only by their sender, so someone can’t put words in your mouth, and it’s always clear who’s saying what.
How is this better than Wave?
This will solve many of the problems that plagued Wave.
To begin with, users can use the familiar Gmail UI, rather than a new, different UI that was extremely heavy on system resources and never worked properly. There’s no new terminology or concepts to learn. You send mails, not “waves”.
You won’t have another inbox to check.
This proposal for editable mails also fits in with how email works than Wave did.
For example, only you can edit your mail, not others. It’s always clear who’s saying what. This how email works, too, and editable mails preserve that.
Different mails in a thread can also have different recipients. Editable mails preserves that property, too, unlike Wave, in which people are added to a thread as a whole, not to individual mails within the thread. The question is not which model is better, but which ones users expect. In Gmail, they expect each mail to potentially have different recipients, and editable mails preserve that property.
In mail, when you add someone to a thread, you get to decide which of the history of the thread they should be allowed to see, by deleting some or all quoted text. Editable mails preserves that property.
Email is also federated — people using different services like Gmail, Outlook.com and iCloud can communicate. Editable mails will gracefully degrade — if one of the recipients isn’t using Gmail, the edit function won’t appear in the UI .
 Along with version history, to prevent people from editing a mail they sent and then claiming that they didn’t say that at all. Perhaps a slider you can drag to see all versions of the mail. Everyone who can read the mail can also use the slider.
 As opposed to letting the sender edit it, which will result in some recipients reading the updated version of a mail, and others reading the original version. This will cause confusion.