- Full encryption integrated into the client for all data (e.g., I don't want Windows Search able to index the mail so someone has access when the client is closed/unencrypted; I also remember testing Thunderbird years ago and was able to go into the individual unencrypted .eml files [I think that's what they were] and read the messages without having Thunderbird opened)
- Full local sync with Android/iPhone (i.e., home WiFi, Bluetooth, or USB cable); it still amazes me that Thunderbird still doesn't have this built-in
- Xobni-like functionality (e.g., showing all emails and attachments to/from sender when clicking on an email, keyword searches); yes, I know the plugin is still available but it doesn't work properly with current Outlook versions (and no plugin like this exists for Thunderbird)
But if the upgraded K-9 also got other integrated Thunderbird functionality (e.g., calendar, contacts, tasks), that would be especially amazing as I could then move away from Google (which I'd also LOVE to do). For example, the Notes field in Google's base Contacts app is limited to 1000 characters. That means I can only sync Outlook to Android one-way (else I lose longer notes when they sync back).
To be able to move away from Microsoft and Google to open source... now that's a future worth dreaming about.