For Gmail and Outlook and probably imap accounts, add the ability to send messages using a server confirmed alias. Bonus points for auto loading the account aliases that are defined for Outlook and Gmail.

allow the signature field to have HTML tags. This could be a simple toggle switch to allow HTML similar to the iOS version.

TeslaUnread shows unread counters on app icons for the Nova Launcher. I would love to use this feature also with Boxer.

create more themes to select from within the app or release a developer api so that other people can create themes and release them on Google play.

Need option for Peak / Off-peak sync schedule for Exchange. I don't want work email pushed to me evenings and weekends

With Boxer Exchange have the ToDo's sync with the Tasks on the Exchange Server. This would allow the ToDo's to be viewed and actioned on the Desktop as well as would allow for syncing across devices.

PGP/GPG encryption with support for Open Keychain integration.

Using FastMail IMAP... drafts are being stored on device... not the server... that makes them useless

Have a counter of unread mail In front of each account like in EE

Can you add facility for rich text editing in compose and signature.
For compose , it should have
1. Bold, italic, color, underline
2. Bulleting, numbering
3. Redo, undo
4. Superscript, subscript

This will allow for more space

For signing E-Mail at least

I often switch from ROM to ROM and other devices. One of my favorite EE features was the ability to back up each account separately.

Integration with Google Calendar allows Android to automatically go into "Do not disturb" (silence) mode when you have a meeting.
This is a feature called "Automatic rules' inside the Sound setting in Android.