Sending EPUB via email is already active. Other than eliminating the need to use a fake file name extension, it works the same way it always did. It converts to both MOBI (azw) and KF8 (azw3) formats and delivers whichever is appropriate for each Kindle app or device.
And despite possibilities raised in this thread I doubt that there will be further change to this.
- It does not convert to KFX format. Amazon uses the Enhanced Typesetting made available by KFX as a selling point for their store. They have no incentive to convert books from third parties to that format.
- There is no sign that DRM will be added to personal documents. It would be pointless.
- Personal documents will not be allowed to contain metadata that makes them more functional than they currently are. Linking to Goodreads, grouping series, and other features provide incentive for purchases from Amazon.