I suspect if dark side (sit or empire) gets a boost in order to be arena viable Palpatine or Vader need a boost as their leadership abilities tie the two factions together.
If Vader got a unique ability (doesn't need to be awesome just have some effect) and a slight change to his leader ability (see below) a Vader Thrawn Nihilus team could be a good start at bringing the empire and sith factions together in a viable arena team.
In Vader's leadership replaced the word damage could be replaced with the words damaged or caused an effect to an enemy in such a way that it included Nihilus' s drain force.
