Star Wars: The Force Unleashed/WMG

In the Force Unleashed II, it will be revealed that Starkiller has been a clone since Vader stabbed him through the back in the first game.

 * In the novel, the question of how Starkiller came back from the injuries Vader inflicts on him (stabbing him through the back then throwing him into space) is brought up a few times but never given a straight answer. Galen no longer has his scars and remarks near the end that he was built "tougher" after being healed. Earlier when he was escaping from the Empirical after Vader gives him his marching orders, everyone was calling him Subject 1138. In the cloning tank teaser site for the second game, the scientist monitoring the Starkiller clone says that "Subject 1138" has returned to the cloning facility. Given the extend of his injuries, the mystery of how they were healed (a mystery that isn't glossed over in the novel), and that Galen was revived a whole six months after he was injured, I'd say it's pretty damn likely Vader was forced to clone him to bring him back and is now trying to make a Starkiller who is easier to control. How else could Vader have made a Starkiller clone in time for the second game (taking place only a few months after the first) if he didn't already have a few on hand from before?
 * Didn't happen.
 * Well, nothing happened in regards to whether Starkiller is a clone or not. We never got a straight answer.
 * One of the cutscenes you can unlock by completing challenges implies, but does not confirm that this may be what happened.
 * One of those cutscenes shows a dead Starkiller in his default-training gear, the costume he wore when Vader stabbed him halfway through the first game. If Vader doesn´t redress the people in his corpse collection in their default-costumes he should have worn white hero robes. I wouldn´t be surprised when this theory turns out to be true.

Starkiller was already cloned, several times.

 * Expanding on the above theory. Starkiller has been cloned several times since before the first game even started. Starkiller has likely died several times and each time Vader awakens a fully grown dormant clone to replace him. A likely twist in in TFU 2 is Starkiller finding all the replacement clones Vader has stashed away in hibernation until this Starkiller dies.
 * So Starkiller is a Venture Brother?
 * This has happened before in Expanded canon, so there is precedent. This happened to Bevel Lemelisk; one of the main designers of the Death Star.

The series is a campaign played by the Darths and Droids players.

 * It was meant to be a stealth espionage campaign. Then the GM made the mistake of letting Pete play a Jedi.

The Starkiller in TFU2 is the original, and after the Dark Apprentice was finished, Vader set Starkiller loose, as part of a Xanatos Gambit.

 * Basically, if the real Starkiller manges to do as Vader hopes, Vader throws the fight, and waits to see what happens...He probably ordered the Dark Apprentice to intervene should Starkiller decide to kill Vader, and ordered him to sneak in if he was captured.
 * The novel more or less gives this some basis in fact, as Starkiller has a vision of the Dark Side ending. As for the Starkiller that the Dark Apprentice saw, it was an unconcsious Starkiller, revived as part of Vader's gambit to find the hidden Rebel base.

Starkiller, Kota, PROXY and maybe Juno were all on Alderaan when the Death Star destroyed it.

 * It would explain why there are no super-powered Jedi, or robots capable of imitating a human during the original trilogy, as well as contribute to Ben's powerful force heebie-jeebies when Alderaan was destroyed.

Starkiller possessed his clone

 * Palpatine did the same thing with his clones. And one of the downloadable costumes for the first game was Starkiller as a force ghost. It would explain any inconstancy with the clone. Galen's also much more inexperienced than Palps, so that may account for why the memories aren't there.
 * IMHO, Vader was telling the truth when he reconstructed Starkiller on the Empirical after he was stabbed and thrown into space. Then, when Starkiller released all of his Force Energy in the explosion that bought the Rebels time but killed him on the Death Star, Vader used Sith Magic to force (if you'll pardon the pun) part of Starkiller (the part with his powers and some of his useful memories) into either a growing clone of Starkiller or his original body (most likely the former), but Starkiller's spirit was so strong that the peice Vader "discarded" came back to him, and just needs time to reorganise itself into Starkiller 2.0. This is why Palpatine's clones (and Starkiller) don't go insane - the insane essence of the clone is destroyed, replaced with Starkiller/Palpatine's. (Palpatine possessing his clones is canon - the power, called the ritual of essence transfer, is first mentioned in Darth Bane Dynasty of Evil). Just my theory, but it explains why Starkiller.

The Bad Ending is the canonical one.

 * Just think about it; Why haven't we seen any of these characters in the original series? Because the entire cast of the Force Unleashed 2 died on that pier, slaughtered by the Dark Apprentice. Vader writes off the whole project, and eventually disposes of the Dark Apprentice at some unspecified point in the future. Continuity is thus sustained
 * The whole game was a plan by Vader; He releases the (clearly flawed) Starkiller clone- I mean, Vader wasn't even trying when he said he'd execute him- to lure General Kota and a significant portion of the Rebel Alliance onto Genosis. The Dark Apprentice kills everyone; Vader doesn't need any more clones of Starkiller, because he now has one that is totally loyal to him. The Dark Apprentice is most likely destroyed, or simply goes missing, in one of Luke Skywalker's Expanded Universe adventures, preferably written by Matthew Stover. Or he simply wasn't meant to have a long lifespan.
 * That...actually kind of makes sense.
 * I sign this under the condition that there will be no third game.

The third game is played from the point of view of Dark Apprentice.

 * It will show him trying to find the rebelbase where Vader is brought to.

The third game is played from the point of view of several Dark Apprentices.

 * The apprentice went with Boba Fett and the Slave One after the Rogue Shadow in the good ending and fights through the rebel base with maxed out force powers, rescues Vader but will be killed in the attack. The rest of the game will starr a new stable clone from the improved cloning project who starts out with little force powers (like in TFU1). Maybe there will even be an optional co-op campaign with two clones like in a more mature LEGO Star Wars game.

Starkiller witnessed the death of Ahsoka Tano.

 * Think about it. Anakin is the type who would want to "take care" of his ex-Padawan himself - turn her if he can, kill her if he can't. Its Personal with him, especially with anyone he actually came to care for (see Obi-Wan for an example). Galen witnessed the fight, was too young to really understand what was going on...but was impressed enough by her to adopt her unorthodox lightsaber style.

The "Vader" that Starkiller captures in TFU2 is actually a PROXY-type droid.
Vader realized the utility of having an army of Starkiller clones, so it's not impossible that he would also consider mass-producing PROXY droids with the intention of making them into sleeper agents. In this case, one of them impersonated Vader for the final battle of TFU2, so that no matter whether Starkiller chose to kill or capture it, Vader himself wouldn't be at personal risk. If-and-when The Force Unleashed 3 comes out, the first mission will involve this PROXY droid managing to work its way out of rebel security and creating subterfuge.
 * That can't work. Droids cannot use The Force, no matter how good their technology. Even assuming that this PROXY clone could emulate Vader's force powers, it could never mimic Vader's force presence, which Starkiller of all people would be able to recognize.