Stock messaging, voicemail, dialer, etc on my Galaxy S4 show a notification on my lock screen which I can tap, unlock the phone, and it takes me to that message in the app.
This is the only feature I miss using Textra; being able to see the contact and preview text on the lock screen itself without needing to drag down the notification bar just to see who it is from and what it says.
This feature (I believe) exists on all Galaxy phones running android 4.3 and up.
If this ability isn't proprietary to just the default messaging app, I would love to see Textra take advantage of it.