The way I see it, there are three major obstacles to making a new race: making armors fit, voice acting for the story, and new zones to go along with the race. So let's look at where the tengu stand in regards to them:
**Armor** Well, the more a new race's rigging corresponds to an existing race, the easier it should be to get armor models to fit on that race. Since the tengu already use the same rigging as the charr, making the armor models fit them should be easier than if they were working on a race that was completely different. Not to say that there wouldn't be any extra work needed to get armor pieces to fit right, but it should actually be fairly analogous to taking an armor piece initially designed around humans, and applying it to sylvari, as an example.
**Voices** The other potential really big issue for a new race, as the story has a lot of voice acting to it. There's two ways you could potentially work around this: First, as some people here have said, you could essentially have it that the tengu start at the current stage of the story, and don't experience any of the story prior to EoD. The other option, which I've brought up elsewhere, is to not actually have a new voice actor for the tengu, but rather use the existing recordings for one of the current races with some post-processing effects to alter the pitch and make it sound a bit distinct from the original. With that, you would have 95% of the voice lines you need for a new race without having to do any new recording (of course, you'd still need to get new recordings for that remaining 5%, but that should be fairly manageable). This wouldn't work for a new race with a distinct accent (like the largos) or an unusual speech pattern (like the quaggans), but for a race like the tengu, it should work fine. And frankly if having two races sound similar is the price to be paid to see a new race happen, I'd be fine with it.
**New Zones** I figure that a new race would require the addition of two or three new zones (not counting the level 80 content making up the rest of the expansion): a new city, a new tier one zone (the equivalent of Queensdale or Ashford), and potentially a new tier two zone (like Kessex or Brisban) to serve as the newbie experience of the race, after which they could reasonably be folded in to the existing zones for the rest of the leveling experience. And of course, people would still expect a full batch of level 80 content out of an expansion pack, meaning that EoD would need to be larger than either of the previous expansions. I have to admit, the fact that the Icebrood content is going to be so thin for the next several months actually has me a bit optimistic on this front, as I could see them doing something like putting the Living World team on making the tengu content, while the main expansion team is working on the Cantha zones.
Just my thoughts on it.