My personal prediction regarding Finn, Rey and Kylo...
Finn - No real backstory. Probably recruited from some Outer Rim planet where he felt he had no future. To be distantly related to say, Lando, would just shrink the universe too much. Finn's story will mirror Han's, in terms of coming into the world as an outsider.
Rey - Skywalker. The way the flashback scene was described, I seem to recall that in the vision Maz gives Rey, Kylo looks at her, and yet ignores her. Due to that, part of me thinks she isn't remembering it, but viewing it as an outsider. That said, I think she'll ultimately be there. We can kind of assume, that up until that attack, there was no real threat to the Jedi. Thus, he probably kept his daughter by his side. Why wouldn't he? After the attack, I wonder if Luke places her on Jakku to keep her safe. It's rumoured that Luke is guarding an old Sith tomb...either he's waiting for Rey to come of age, or he's waiting for a resolution to the disturbance in the Force he's probably felt. What possible reason can he justify leaving his daughter somewhere remote, AND turn his back on his friends and allies? I'm sure VIII will explore this more, but I am looking forward to THAT flashback more than anything I think. Incidentally, I think they could show Luke in that scene, but never reveal his face. From behind/afar, they could quite easily depict a cloaked figure with a green lightsaber, or, most likely, I think that scene will show Luke discovering the aftermath of the attack.
Kylo - It's all but certain he's a Solo. He's obviously force sensitive too, so Leia would have felt that in him upon his birth. She either helped guide him, or sent him to Uncle Luke for training. Ambitious, with an incredibly strong bloodline, I can see young Kylo being hungry to learn, to be strong, to be better than those around him. I think his ability of the force makes him vunerable, and from the trailers/leaks, it seems Snoke is in his head at the time of TFA. I think Snoke got into his head at a far younger age, tempting him with the power of the dark side and the legacy of his Grandfather. He abandons Luke/Leia and the path of the Light Side, and succumbs to the dark ways, and follows Snoke's guidance in joining/forming the Knights of Ren. The attack on the Jedi is an attempt at wiping them out, including Luke. After Luke goes into hiding, Kylo makes it his goal to kill the last remaining Jedi.