I personally like the idea of the Sith Inquisitor, because he can add content to the idea of underworlds, the simple civilizations on the planets under the reign of the Empire (where the Rebellion rises from), I don't see him as a Sith , because there can only be two right? We already have Sidious and Vader, I see the inquisitor as a pawn of the Emperor, like with Hitler's Gestapo , in charge of a secret police.He's a Sith Apprentice or a Dark Jedi who has the ambition to fight Darth Vader one day to become the second Sith, as he's loyal to Only Sidious.....or Vader , as Vader might have a secret agenda of his own aswell.Nevertheless , there is a Jedi-purge going on after all, so the additions of extra forcewielders on the side of the Empire wouldn't feel that far-fetched to me.I'd like to think that a lot has happend , untill we get to the scene in "A new Hope" where Tarkin addresses Vader , about the Jedi.......
I like the idea of using officers, like hit-squads, the samekinds that were used in ancient rome and during the Spanish Inquisition, or like somesort of SS devision, not literally ofcourse.So the rebels have something to fight against.
These events could explain the rivalry between Tarkin and Vader more clearly, as Tarkin sees more potential in using hit squads against the Rebellion and Vader failing in the use of an Inquisitor, letting him take control as the only Sith forcewielder next to Palpatine in the Original Trilogy.