We've already found out. That's Adam Driver talking. Not Mark Hamill.
Where did we find that out? (I hadn't seen or heard that, but I also have purposely avoided some information.)
With all due respect, Alio, I can't think of a worse theory. Luke is as pure a Campbellian hero as it gets. In the face of all the temptation in the world, he defies the Emperor and his own father and holds true to the light side of the Force. He is the prophesied one who will bring balance. He risks his life and is willing to die to show his father still has some humanity and goodness left in him. He watches his father gain a small measure of redemption at the end of his life...
...and then he says lol freak it and decides to help his dad finish what he regretted doing to begin with? That would be M Night levels of terrible. It would be a twist for the sake of having a twist and nothing more.
Fair enough. As I said it was me just theorizing. Luke isn't shown on the poster, but he's credited. Luke is only seen for a brief flash in the trailer. We see a lot Ren.
Ren only says he aims to "finish what you started." That could be something as direct as "bringing balance to the Force."
That said, Luke could just be the next trilogy's Ben/Yoda. I don't know. Like I said, my theory is that he's Ren.
EDIT: Okay, so I just saw I'm not the only one who has this theory and it's being disputed in various places (I also just saw that Driver was previously announced as Kylo.)