Ability to delete all the words of an unsent message. many times I have been in the process of responding to someone and they call.
I abandon texting and answer.
the message is saved as a draft, which is good.
If it is a looonnngggg unfinished response I have to either hold down the backspace key or I long press a word, then options to select all and then hit backspace.
As an example, Gmail on Android has an option in the hamburger menu of an email composition screen to 'Discard' if you don't want a draft hanging around.
Textra is the best and I've recommended it to many many - many - people.
I just think it would be a good addition to the already great application