This lack of the main character of the original trilogy has lead to some very vocal speculation that Luke Skywalker has fallen to the Dark Side. Some people have, very foolishly, claimed that Luke is Kylo Ren (despite the fact that we know unequivocally that Adam Driver is playing Kylo Ren, a fact confirmed by pictures). Others claim that Luke is perhaps the leader of the Knights of Ren, or perhaps some other leader of The First Order.
All of these theories are absolutely wrong. Not only do these theories contradict what was presented in the original Star Wars trilogy, but it goes against the information we've been given in the Force Friday books, and the Shattered Empire comics.
Let me digress for a moment.
It's easy to see why this proposed ending provides such a sticking point for Dark Side supporters, but it seems fairly clear that this was never a seriously considered ending. This quote very much sounds like George Lucas throwing out the most absurd ending he could think of, which is why he dubs it "the ultimate twist". Kasdan affirms this with a sort of "well of course!" response, that again doesn't seem like legitimate support.
Now, obviously we're dealing strictly with words on a page, with no inflection, the absolute lack of foreshadowing of a fall for Luke speaks to the fact that this was not a legitimately considered ending. Every twist, whether isn't Norman Bates being Mother, or Bruce Willis being a ghost, must be foreshadowed in order for it to make sense within the context of the story. There is simply no foreshadowing to Luke falling to the Sith. At every juncture, Luke takes the path of the Light Side, even of it's not the path that Yoda or Obi-Wan would have liked.