I doubt she is anyone other than herself with a link to one of the big generations of heroes across the films. A re-incarnation would feel very....cheap to me. After 30 years, Anakin is re-incarnated as a girl. No I can't, and won't buy that, even for Star Wars. We've had Anakin's journey and it's over - he doesn't get a second chance and we deserve someone new; Rey.
The fact Rey has a gift for fighting, or can use the Force after a while, and Leia looks at her in a way that means something? Please. This is called "clutching at straws". She's learnt how to fight because she has no-one to look after her. She knows how to pilot because she's lived around ships and junk and spacers for her teenager years. She uses the Force well because, unlike the older Jedi, she doesn't make it complicated and simply lets it flow through her - like Luke did deflecting blaster bolts on the Falcon and destroying a superweapon in the space of days. And if Leia looks at her warmly, maybe it's because she is the closes link to Han and Luke she has left, and she was with her husband when he died and now considers her family?
Sorry...I just hate these "theories" when it's just people linking what they can from TFA to the prequels. Why JJ and Disney would go back over 50 years in the timeline to continue characters we thought we'd left behind doesn't sit right with me.
I love the fact we can't be sure who yet, and that's just what I'll be back to find out more in EpVIII.
Rant over.