It would be very cool if you could add a small toolbar in the compose window with two checkboxes showing whether the mail should be signed and/or encrypted.

This has two advantages:
- one can quickly enable/disable signing or encrypting a mail (though this can also be done using the menu and using shortcuts)
- more importantly, one can quickly and easily view the current settings

Especially the second advantage is important in my point of view as it would bring a visual indication that the setting was changed when you enable/disable encryption using a shortcut for example. And when answering to an encrypted mail you can see that your answer is encrypted as well, faster than by checking the item in the menu.

(Sylpheed has this mini toolbar and I miss it)