TFU3 endings I've had in my head
Darkside Ending: Starkiller defeats the Dark Apprentice, but is mortally wounded, before he dies he inadvertently betrays Yoda's existence and location. With Starkiller and the Apprentice dead Vader emerges from the shadows and orders the Devastator to proceed and intercept the Tantave IV, while he takes care of a "priority matter."
DLC will be Vader based
Lightside: After defeating the Dark Apprentice Starkiller goes to Juno, as they embrace we hear the sound of a lightsaber activating. Juno stabs Starkiller with his own blade, Vader emerges from the Shadows and congratulates "Major" Eclipse.
It turns out Starkiller is the genuine article, Vader reveals that he's had doubts about Starkiller's very loyalty from before the mission to assassinate Kota. He arranged for Eclipse (in actuality one of his most trusted agents) to "test" where Starkiller's true loyalties lie. It was win - win. Either Vader's doubts would be silenced and he'd be confident that Starkiller was worthy, or Vader's most trusted operative would be highly placed in the (now unified and easier to destroy) Rebel Alliance.
Starkiller dies discovering that he was used by the Dark Lord from the beginning. Juno and Vader board the Devastator, she gives Vader technical data on the Tantave IV and informs him of the vessel's passenger.
The Vader Based DLC would take place on Dagobah and Tatooine
With Dagobah Vader has to move through the hostile swamps and that cave (where ha'll fight his Jedi self) before reaching Yoda.
With Tatooine, Vader on catching up with the Devastator learns that according to the Tantave IV's manifest, the Droids "they are looking for" are an Astromech designated "R2-D2" and a Protocol designated C-3PO, he takes 'personal' interest.
He tracks down the Sandcrawler and fights his way through the Jawa's (turns out these little b-stards have an army of CIS battle droids for security.) After defeating the opposition Vader interrogates the Chief Jawa and learns where the droids were sold.
"Uncle Owen, Aunt Beru!"
"Luke, I am your Father."
With his son now safely in his custody Vader orders the Sandtroopers to double back to the Sandcrawler, where we have the boss fight to end all boss fights. Kenobi can not allow Vader to recover either Luke or the Death Star plans, so the 'old man' is going to be damned hard to kill and will have the aid of Yoda and Qui Gon force ghosts.
The Republic will be reorganized into the first Galactic Empire!- Emporer Palpatine