While I don't know if they'll go this route, and don't know what sort of dilemma they're going to go, I think it's very notable that at the end when Rey excepts the force, she draws her power from the dark side. When she completely dispatches Ren, she is clearly doing so out of immense anger (over his killing of Han, his injuring of Finn who at that point she doesn't know is still alive, and her attacking of him). While it's contrived for the two to be perfectly separated at the end by the planet's destruction so cleanly, I think one of the most notable things that hasn't really been touched upon greatly in this thread is that not only does it ensure Kylo's survival, but it also prevents Rey immediately slipping into the dark side more notably. If they weren't separated, based upon how they were acted and how the scene was directed, there really seems to be little doubt that Rey would have simply executed Ren while he was defenceless. I know the novelisation seems to portray the scene differently, in that she consciously chooses to allow him to live, but I think still having the lightsaber extended in the film, and slowing her advance towards him once they begin separating, is very suggestive that she'd have simply outright killed him. Now, whether people view this as suggestive of a 'dark side' like action is something else entirely, because after all I would imagine that the Jedi killed Sith at numerous points, but my thought on the matter really just draws on where she drew her power from in that moment, and Ren being defenceless as he laid on the ground awaiting her advance.
I think it could be an interesting direction for them to go for both Ren and Rey to feed primarily off the dark side of the force, and have one try stop the other, in how it distorts the cinematic universe's view that the light side is good, and the dark side is evil, but I think they will avoid having Rey ever truly using the dark side or becoming villainous (unless it's in 10/11/12) due to the significance of her character (from both her position in the new trilogy, and the significance she plays being the first female lead in a franchise and a desire not to have her become dependent on a male or have her position downplayed). I'm kind of curious how they'll progress regarding both Ren and Rey in relation to the force. If they go with Rey being tempted by the dark side continuously but managing to overcome it, they end up with a very similar situation to Luke. Maybe they'll go with a very neutral divide where she draws from both light and dark sides of the force to be a much more balanced individual than we've seen before. Maybe she'll become a dark Jedi. Maybe they'll just forego the idea of the force having sides and will just focus on her emotional and physical journey throughout the idea and won't bring tension in from how she draws on the force, but rather how she uses it. I just find it interesting to see which way they'll go, as the most obvious (being tempted by the dark side and overcoming it) has somewhat of a 'been there, done that' sort of feel to it, but that's not inherently a bad thing if it's executed well.