Come now, Rey is NOT going to be the bad guy (or gal). She is clearly the main hero of the new trilogy. This is also in line with the general cultural trend of re-writing/re-working major stories to incorporate female and/or minority protagonists. Actually, consider that the only white guys in TFA either die (Han Solo) or are bad (Kylo Ren) or old/not really in it (Luke). The main characters are female (Rey), black (Finn), and brown (Poe). I don't have a problem with this, mind you, just pointing it out.
As far as Rey's parentage, there are really only a few options from most to least likely:
1) Luke. I mean, it is the most obvious answer.
2) Han and Leia. I'd say only slightly less possible, and would fit the general TFA trend of aping the original trilogy..."Rey, search your feelings...you know it to be true: I am your brother." "Noooo, that's not true!!!"
3) Obi-wan. This is least likely because of the age gap - Rey'd have to be 30+ years old, which is obviously not the case. On the other hand, daddy could've been a force ghost when he got it on with mommy.
4) Some other. We'd probably have some clue who it might be, and there's been no easter eggs as far as I know. Maybe Boba Fett blasted his way out of the sarlac pit and settled down and got married? Maybe she's Obi-Wan's grand-daughter?