Bonus Mission Pack
I think it would be better for the next expansion to actually add Quaggan and Skritt to the playable races and make an alternative story for this races that differs from the main story of the other classes and shows funny and unexpected sidestories to the main storyline or living story content. I think it would be too big a thing to “waste” on a free update. To avoid having to adapt all the armors to the new models there would have to be new costumes for them though.
With the popularity of the Quaggan I would imagine, that they would sell a lot of characterslots and make a ton of gems with different hats and costumes in the gemstore.