It's not about 'fully fleshing out' the character, per say - honestly it's sort of the opposite for me. I feel like Rey is already basically fully fleshed out. I don't picture her having to struggle and learn how to use a light saber. It doesn't seem like she'll have any problems picking up force tricks/magic either. She seems to already be proficient as a mechanic, with a more fundamental understanding the Millenium Falcon than Han Solo did, she's acrobatic as fuck already, so I don't imagine that'll need to be worked on a lot either. She doesn't have any significant character traits that make me worry for her growth - she isn't remotely a coward, she doesn't seem to have an immoral bone in her body, so it's difficult seeing her go dark side. She misses her family I guess, but she seems to already be 'over' any downsides that issue had (she isn't going back to Jakku). Maybe she's a bit overconfident - but even then, she's crazy humble about how baller she is - she saves a dude from a crazy octopus alien with amazing door closing timing, and she pretends like she had nothing to do with it.
This is basically the problem for me. It's like she's already 'end goal' character. Finn, we saw his serious flaws in this movie and we also saw him have to fight to get over some. And now he's mad fucked up - so next movie we have so many options with his character. Is he force sensitive? If so how powerful? Does he get more brave and less spazzy? How bad are his injuries, are they gonna impact him in any way? What does he plan on doing now that empire stuff is over? What does he want to do with this life, who the fuck knows right now - but as an audience with a trilogy coming up, we can wonder about this and have really interesting conversations about where he might go. Ben/Kylo Ren - similar, but maybe not as much fun stuff. He's going to train with this mysterious sith we know nothing about - which could be interesting. He'll have to deal with killing his dad thing, maybe he'll feel bad, I don't know. Honestly, not as cool as Finn but you can maybe see where I am going with it.
With Rey though? She's an incredibly strong Jedi, and pretty sure she'll be the strongest. She's going to probably train with Luke in the next movie, and other than that I can't really think of anything I can picture her doing. Maybe looking for her family? Her mysterious family (which I am pretty sure is not related to the skywalkers) could be an interesting lead - but honestly I wasn't titillated enough by that - I know too little there. She's not going to have to become a better person in any way that I can think of, her personality is balling. I can't see her having any trouble with training. I can't see her being tempted by the dark side, she's crazy emotionally stable, all things considered. No injuries, no significant recent losses. They sort of just thrust her somewhere with all signs pointing to something pretty expected happening - her being trained by Luke.
All this critique makes it sound like I hated the movie, but no I really enjoyed it - I am going to go see it a third time probably this week. I even really enjoy Rey as a character, but I don't like what they did with her in this movie. I think they did a much better job with other important characters, and a few equally if not worse jobs (Poe) with some others.