So we all know that it would be hard to get a new race that also gets a playable main story line, so here is my idea about how it could be set up.
You play as your Tengu in a vision of the past, I'll explain it better... You are able to play your new cool Tengu character through the main vanilla story but you are living it as an out of body experience, where you act as the commander but you hear the standard human voice when the character is talking.
As an example in the battle of claw island mission you would see your Tengu but in the dialogues you would see your character not speaking and instead hearing the voice of the human commander.
Another solution that would result much cheaper but I would still love it is applying a filter to the human player character voice to make it sound like a bird, like they do with the charr voice with all the little grawls and stuff, and that would be enough!
Also I would add that about the armors it would be easier of the devs would build the Tengu to fit the same pose/skeleton of the charr so we could have the same amount of armors and the devs would save time(I'm guessing the devs already thought about this since is common practice in game development but I wanted to get it out there), we would get clipping as with the Charr but I think we can all learn to live with that.