I keep seeing these stupid posts concerning Luke Skywalker from The Force Awakens claiming Kylo Ren is actually Luke Skywalker. (1, 2, 3, etc.) Seriously? This is demonstrably untrue! Luke IS in the trailer, and he has a whole voice-over in a nice calm voice. Sure, he is wearing the dark outer cloak, but the under cloak appears to be the typical old Jedi garb that his own mentor, Obi-Wan Kenobi, wore. And it might take place with his, or Han and Leia's possible children- but I think it's Rey and Finn he is talking to, but I can't discount the fact Kylo might be a child hearing it from him either if it is from the past.
Should I also mention the fact there's been plenty of pics of Kylo Ren without his helmet, showing Adam Driver's visage? And for the people thinking Luke might be evil because he isn't on the poster forget that Obi-Wan was NOT included on the A New Hope posters.
I can't say that he won't be a villain in some capacity, but he is certainly NOT Kylo Ren. Personally, I think Kylo Ren is of the Skywalker bloodline, more probably a Solo, being Han and Leia's child. (also note the first part of his name Kylo- S-KY-walker + So-LO... just saying...) It would make sense in the Star Wars film saga being a chronicle of the Skywalker legacy to have him be one. I mean, I also think that Rey might even be a Solo as well, but that's another story.
Back to Luke- I think he has gone into self imposed exile as a way to preserve the Jedi. Following in Kenobi's footsteps, maybe he's gone to guard something. Or I've also thought that maybe, being the only true remaining Jedi as well as being the son of the most powerful Jedi that ever lived, he may have gone into hiding to protect others from an almost uncontrollable amount of Force power. (Note: Disney scrapped the Expanded Universe, and in Aftermath Luke is mentioned as being possibly the only one left)
As for Kylo, he clearly is following in the path of Vader. He's already trying to imitate him with the mask and suit, but what training does he have? Pretty much nothing. What if Ren is a wanna-be Sith? He doesn't have anything other than the stories of Vader to go from. No master to follow. He has a small grasp of the Force and built his lightsaber to emulate Vader's- Or from Vader's dropped lightsaber, as my friend suggested- to which I responded that would be a great mirror to seeing Luke's old saber (also from Vader/Anakin) from the trailer as well- but it looks like it might not be quite right and he hasn't got an instructor to guide him. It has a weird unstable appearance, which makes me wonder if the hilt is actually an exhaust of some sort. Like Ren couldn't get it to work properly and made due.
However... with those ideas, I don't discount the idea that Luke may have turned towards the Dark Side. That is always a possibility, no matter how doubtful it may be. Though, I also think that if Luke and Kylo fought each other, I fully believe Luke would crush Ren without a problem. Ren appears to use fear to his advantage, whereas Luke at the end of Jedi was not only a competent Force user, but a confident Jedi Master and would see Ren as a minor threat. The equivalent of Luke from A New Hope going up against Vader right away in the first film- it wouldn't even be a close fight.
Overall, I think Luke is the new Obi-Wan, serving as a mentor to Rey and Finn, to combat Kylo Ren. He may even become the new Yoda- the teacher for a future generation of Jedi. Which might be a very important thing here. These new Force users have no teachers outside of Luke, either good or bad.
Sorry this is very rambling, but you get the gist of it.