I have tried all of the texting apps, each of which had a fatal flaw... except Textra. Though not perfect, it does come close.

Here are a few things that I would like to see.

Text input:
Add the option to customize the text input box separately from other areas. To include the color of it, and adding and adjusting the color of the outline of the box.

Privacy:
Option to make individual messages locked. Hide individuals' last message from being visible in the list view. Private inbox, accessible through a hidden means... such as sending a message to "Unlock0000" where the zeros represent a user's predefined pin code.
The hidden messaging feature would be more appealing should it not be mentioned in the app description, rather the feature should be highlighted upon first opening the app.

Chat heads:
Facebook messenger got it right. Follow that example, using Textra customization options, to easily differentiate between the two apps.
Yes, I could use Messenger to handle my sms/mms needs, but here is why I don't... First and foremost, I would be shown as 'Active' on Facebook... Second, the colors Messenger uses, purple and blue are too similar to my color blind eyes to quickly differentiate between a Facebook message and a sms... Third, all sms and Facebook messages are poorly organized within Messenger.

Pinned icon:
We can change the icon color for Textra, that's great... but we can't change the title beneath it. Allow it to be customized or changed to "Messages"