What should Rey’s purpose in the wider saga be?
I’d just have her as Luke’s apprentice/niece from the start. The newly defected Finn meets her while she’s on a mission.
As for the wider saga, she’d be basically the heir to the Skywalker legacy like she was in the ST we got. Luke would be the Jedi grandmaster who’s training students, but he’d name Rey as his successor to lead the Jedi when he’s gone. There wouldn’t be any actual Sith involved in the story, so the Chosen One prophecy is still intact. And I’d go out of my way to more fully distinguish any Force-using villains from the Sith, both culturally and ideologically. Rey helps defeat them, helps maintain the New Republic, and so becomes the protector of what her family has helped build. Then she gets together with Finn, and so through them, the Skywalker line continues, and you’re left with the impression that the family will continue to preserve the peace they’ve created for the generations to come.