By far the most important: A desktop client that works without a smart-phone. Identity management that is not smart-phone/phone-number based. Better UI for lo

By far the most important: A desktop client that works without a smart-phone. Identity management that is not smart-phone/phone-number based. Better UI for long form messages, including citing previous lines. Better support for adding people to ongoing conversations. UI for conversations per topic rather than per person. Mailing lists, and the ability to filter them to different folders (or equivalent UI elements). Functioning as a searchable database of documents I have received. (Important detail: Attachments aren't independent messages: They are attached to often long form messages that describe their content). Basically, WhatsApp does a subset of what EMail does but dramatically better and encrypted. But that covers only half of my email usage at most.