I know you guys don't plan on supporting Google Voice since Google hasn't really made integrating with that simple or straight forward (not worth the effort, I don't blame you).
The biggest issue with those of use using Voice as our primary number is that Google is making it really difficult to use as the default. Unless an app supports carrier SMS/MMS functionality, it can never be the "default" for system-wide messaging. This is really a problem as the Voice app will never get default status anytime soon, and no SMS/MMS apps can integrate well GV. Their own Hangouts app did, but now it is losing SMS support and the ability to be default along with it, not to mention Hangouts isn't anywhere near as nice as Textra.

So here's what I'm thinking could be possible: Allow us to use Textra for our carrier messages when they do get used (carrier notifications, people who are close enough to actually know our carrier numbers, etc), but allow us to select a different app for outgoing message requests.
For example, when I click on the msg icon from a contacts screen or missed call log, and Textra is the default, instead of opening a new Textra screen allow us to select another app like Voice or Hangouts to send it from instead. We can always go into Textra directly to message from our carrier and of course use it to manage all our carrier-based messaging, but still have this much-needed way to default sending from Google Voice without you guys actually needing to do any Google Voice integration.
Plausible, or am I insane for thinking this is a good idea?