The first and most obvious thing is that, so far, Luke and Kylo Ren have never been in the same place at the same time. Sure, a lot of characters were never seen together. Boba Fett and Greedo never shared any scenes, but no one is suggesting that Fett is a returned to life Greedo zombie seeking revenge (not yet, anyway). But Luke and Kylo are really never seen together, even in promotional material. We know that Luke is going to be in the new movie—he was heard talking in the first full-length trailer, and a very brief glimpse of someone we think is him putting his hand on R2D2. He's the only main character not to appear on the poster for the movie, which is odd. One great reason to hide Hamill is because, well, he's Kylo Ren. We all know what Hamill's face looks like. It's not a spoiler. How different could he possibly look? Even if he grew a beard or got a new haircut, it's not like that can possibly be a huge plot point. They had better be holding off on showing Luke's face for more than "just because." 

The last time we saw Luke in Return of the Jedi, he had started wearing a lot of black. He was even wearing dark gloves when he was on Tatooine. Who wears gloves in the desert? The only part of his body that wasn't covered in black was his face. Well, guess who dresses that way, even on his face? That's right: Kylo Ren looks like Luke just stepped his game up and added a mask. It makes sense, because look at the evolution of Luke's style. The further he got in his Jedi training, the more monochromatic his outfits became. He doesn't have time to match colors. He likes single, solid colors, and black is the most solid color. He started wearing the hood and mask to save time on hair combing, shaving, and brushing his teeth. He's just too busy for fashion!

Lightsabers are super sweet, so they must be really hard to make. If not, then everybody would have one. They're cool looking, they're super powerful, and they're much more effective than blasters. So far, the only person we've ever seen make their own lightsaber is Luke. He built it after he lost his father's saber when his hand got cut off. Clearly, Kylo Ren has a custom built model with the added hilt. Since lightsabers can't just be bought at Space Wal-Mart, it begs the question: where did this come from? There's two options. First, Kylo made it because he's Luke and he's done it before. Second, Luke made it for Kylo, which would be super weird. It's got a red blade, which are exclusively used by Sith Lords. If someone asks for a red bladed lightsaber, they're basically asking for weapon to kill innocent people with. At this point, Luke had better be Kylo Ren or else he's got some explaining to do.