Personally, I don't know why I was thinking this, but I think that they should create two special new classes...unfortunately, I only have one suggestion. The night-sisters for the Sith Empire. Anybody got better ideas. Please share, I'm quite interested.
The Witches of Dathomir, of which the Nightsisters are a clan of, aren't banished to Dathomir until 600 BBY, so they don't exist in this era. BW could retcon them a la the same thing they did with the Chiss, but this is a much larger retcon. Now add in the fact that Nightsisters would be limited to 2, maybe 3 of the playable species(if they stick to canon) and I don't see this flying.
I'd love to see Mandalorians as a Darkside Character
Mandos aren't a species,nor would they be a class. They are a clan made up of different species all operating under the Mando ideology, roll a DS BH if you want to see this. Perhaps you meant the original Mandos, the Taung as a playable species. They are thought to be extinct or very close to it in this time period and the years following it.