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.
"There is something going on in time and space, and beyond time and space, which, whether we like it or not, spells duty." -Winston Churchill
"For of all sad words of tongue or pen, The saddest are these: 'It might have been!'" -John Whittier
"Apathy is death." -Kreia