I can really process email quickly on Airmail Mac by using Gmail Keyboard Shortcuts. If I could use Gmail Keyboard shortcuts on the iPad using an external bluetooth keyboard then that would be something that I haven't seen on other iOS Mail apps.

can start directly an email from iOS8 Notifications Center. Just 3 textfield, like to: subject: and content:

Categories for email/tasks for quick search and reference

The Malto mail app on the iPad has a very neat and different interface on the iPad, which I really like!

However, its functionality is not as clean and simple as that of Airmail on the Mac.

If you could somehow manage to merge the look of Malto with the functionality of Airmail, that would be great!

A toolbar with 3-4 Space you can customise by existing folder or intelligent folder.

A custom iOS keyboard to use easily the markdown syntax( access to [],*...), and custom gesture to set font style on text,, like double tap on a word to set it in bold, triple to italic.
Something like that !

I generally do not want all of my email stored locally. Perhaps Inbox and Starred folders (or selectable) could always be kept regardless of age. Otherwise, all emails older than an account specified range would be removed from local storage.

There is a huge lack of signature and encryption capable email clients in the AppStore when it comes to GPG/PGP.