We've not seen Luke clearly, not even once. He had a voice over in the second trailer, and a half shot implied to be him with R2-D2...but whilst everyone else has appeared he hasn't. This trailer has solidified my theory.
Kylo Ren is Luke. That's my theory, and I've seen no spoilers or anything, so just a guess.
My theory is that either at the start of this movie or in a flashback we learn that Luke fell to the Dark Side and there is something they need to fix. I posited time travel at one point, that's possible, since JJ loves that. The other possibility is alternate dimensions.
Either way, there has to be a reason they are still fighting these neo-Imperials 30 years later, and I think it's because the Rebels were beaten with the aid of a turned Luke and so Han and Leia are playing the Obi-Wan role for their daughter (?).
We'll see how wrong I am in 2 months.
