How about if Rey was born like Anakin was? What if Rey was born with an overabundance of midichlorians and she is the chosen one of the prophecy? It would tie up a major plot hole in the prequels. This also keeps open the possibility of there being a relationship between Kylo Ren and Rey , which she ends up being either his redeemer or his sworn enemy. If Luke had a son or a daughter, either he/she hasn't been introduced, or more likely that person played an important part that turned Kylo Ren into a Sith. When Anakin cut down Mace Windu, he fully gave himself over to the dark side. There was a similar moment in the current movie, but Ren at that point, had already given himself over to the dark side.