OK so final theory time.
1. Rey and Finn are both Jedis. Rey is a Skywalker. Finn I feel like will be more of a Grey Jedi, susceptible to perhaps the Darkside at time. A lot of people speculate that Finn isn't a Jedi, he's just got Anakin's old lightsaber, I think they're both Jedis though. I think the Knights of Ren present a foe where multiple Jedis will be necessary to battle.
2. Snoke is not Plagueis. I know this is popular, but I think instead he'll be a second disciple of Plagueis. Acting more like Plagueis acted and hid from Sideous knowing Sideous would kill him.
3. Vader is a martyr for the First Order. Explains what was said in the trailer by Ren. Vader was seen as a great leader to many and I'm sure many think that the Rebels killed him in cold blood.
4. Someone big is gonna die. Not sure who it will be. But someone big will die.
5. Luke is in exile similar to Yoda and Obi Wan. But the Knights of Ren attacking his friends and family draws him out. This theory is based upon the idea that Luke became super powerful but was feared of turning, or perhaps was coerced into turning to exile because everyone was afraid of his powers. I don't consider that last part likely.
6. Lens flares everywhere. Get ready for it. I dont' think it will be as bad as Star Trek, but Abrams loves using Lens Flares so be ready.