Now if Luke is a Sith, he is either Kylo Ren, or a different, new Sith. It seems to make the most sense that if Luke is a Sith, that he is Kylo Ren, throwing in another bad guy wouldn't seem likely. I actually looked up pictures of the new Lego Kylo Ren minifigure, but there is no human face under the helmet, which gives the Luke theory more evidence, but at the same time, lack of it.
Kylo's words in the trailer definitely gain much more significance if Luke did in fact, turn to the dark side.
Overall, I'm not sure, but it is definitely plausible, and I tend to think it is right, though I do not know for certain.-Andrew