I may be completely off my mark here, and if so I apologize.
I personally believe the most likely scenario for a new playable race is with the introduction of an entirely new region, similar to HoT but on a bigger scale. If done this way the personal story and leveling zones can all be tied into the story up to that point, integrating the new character into the current story without having to backtrack through multiple expansions for the story to make sense like other mmo’s.
As far as how to treat existing story with the new races, I think the best solution would be to bite the bullet and voice everything over. However, more cost effective options exist – the most immersive being to write in an excuse to send the race back through the old story, you will see your character but the world will see you as another race. Could be explained as “you are late to the party, you should know how we got here” or something. I believe this would cut down on the voice acting significantly. Another option would be to simply not have any pre-xpac story be voiced for the new race, or even release pre-xpac story episodically for the new race.
As for the work involved in retroactively fitting all old skins to the new race, I think the work actually depends a lot on the race chosen. Dwarves wouldn’t be too much work relative to another anthro race like the Charr, but I think the overall work of implementing a race that was never at any stage of development that we know of is too great. Tengu are the most likely imho, and I think there’d be far less work involved than people might believe – looking at their model types they share a lot of similarities with Charr models: an elongated facial structure, reverse leg joints, a sort of hunched posture, etc… But more importantly, let’s not forget that the Tengu were planned, so we don’t know how much vanilla work is actually done; they could be practically ready to launch but Anet decided to keep them as part of a more cohesive grand plan.
To summarize:
- Dwarves and Tengu are my top 2 choices for likely race.
- Would probably come with a large map expansion, including new leveling zones
- Would integrate into current story at the time
- Would have a creative mechanism for experiencing old story as an optional endeavor ( optional = immersion not #1 priority)
- Dwarves skin modeling might be the easiest from a labor perspective
- But Tengu may have most of their work done already
Overall chance of seeing a new race in my mind: 70%, a lot depends on the success of the gem store prior to announcement of next expansion. They seem to have entered a sort of cruise mode on the gem store with a lot fewer variety of new items and more re-introductions. I believe they’ve figured out a good system for gem store items that can sustain them while only dedicating the bare minimum development team to it, freeing up labor for other projects. If we start to see a bigger push of new items then I’ll lower my chances.