I think that Sunset Shimmer is an element... Probably the Element of Inspiration. Because at the end of EG2 she was apart of the rainbow that the other six formed. I know it's not that much of a reason to think that but she did inspire Twilight quite a bit during the movie.
Also I think that Changelings want to love and they can fall in love, they just can't show their feelings because love is their food source and they will just drain the energy of the pony they love. I also ship Chrysalis and Shining Armor.
And I hate the idea of Fyrefly or FireFly being Rainbow's mom, so I think she just has a father as a parent.
As for Applejack's parents, I think they died in a carriage crash making a delivery or something, shortly after Applebloom's birth.