A fan named Cybsled shared his own fanfic story (speculation) to explain why Eredar would be a digestable new playable race. It would make sense:
"Blizzard's April Fools joke wasnt a total joke.
I've seen the grabs of the E3 video, and the starting area clearly looks like Mt. Hyjal.
My theory: After the wisps all owned Archimonde, they absorbed part of his power/energy/form/whatever. This overwhelming power transformed the wisps, giving them mortal form again in the shape of an Eredar. However, since they have the souls of elves, they arent evil and are accepted into the Alliance on this fact.
This idea fits right in with the Blizzcon statement where Metzen said "At first you will go "HUH?" then go "Oh, that makes sense".
Yet what we would need. Another Elf spin-off ... I remind you this is not official. And has not been confirmed to be a real screenshot. Specially since bigger networks such as IGN, Stratics and Worldofwar.net have not even shown any screenshots of the new race yet -- it makes you hesitate.
Links
worldofwarcraft.com - Source