I still go with the daughter of Hans and Leia for reasons I stated earlier, but your comment does raise some interesting ideas.
Specifically Luke or Leia going to the dark side.
Luke would be too obvious, repeating his father's fall, but hey, JJ Abrams didn't seem to have a problem "remaking" A New Hope.
Leia raises a more interesting possibility.
Strong emotions are the path to the dark side and it wouldn't be that hard to have her get pissed off over the death of Hans.
Wanting revenge, she goes bad ass and slips to the dark side.
Now you have a multifaceted conflict.
Luke vs his sister and nephew.
Rey vs her mother and brother.
If done right that could be epic.