Rey Jade - Daughter of Luke Skywalker and Mara Jade, former hand of the emperor. Could be they were married, could be it was a fling and Luke didn't know about her. Could be it was serious/committed relationship and Mara left when she realized she was pregnant, fearing that she may raise the next Vader. Luke's long-term bitterness at her disappearance distracts him in the training of the next Jedi order, allowing Ben Solo to slip to the dark side and inevitable betrayal. Mara leaves Rey on Jakku in desperation as she is fleeing a strike team led by Snoke, a commander in the imperial remnant and its newly formed First Order. Mara is killed in a battle with Snoke, leaving Snoke heavily wounded and permanently disfigured.
Would be the most delightful nod to the EU, and provide plenty of fodder for non-movie content (books, comics, etc). Also... Rey Jade is a pretty dope name, despite being a tid bit strippery.