Since they seem to have "Galen" coming back to life, I see nothing wrong with having him fight a resurrected or ghost Sith Lord; it's no less ridiculous.
Just for the hell of it, I vote for Darth Revan. I want to see Revan get the **** kicked out of him by Vader's apprentice.
"Grant Allen [...] had written a book about the Evolution of the Idea of God. [...] it would be much more interesting if God wrote a book about the evolution of the idea of Grant Allen." ~ G. K. Chesterton, The Everlasting Man