As of the moment everyone seems to be of one faction. However, this faction is diverse and it would not be impossible for characters to have dark-sided motives, just the same as light-sided motives are possible even though Revan acts in defiance of the council. However, it doesn't seem that having a full-fledged Sith characters would be likely. We need a much larger group for multiple factions like that. Sorry. (Typically works best with one GM per faction)

Though it is possible this could proceed into the Jedi Civil War where Revan's followers could split sides.