tbh i think there's a lot of great characterization in "before the awakening" that the movie kind of side-steps.

for instance, in the book rey's biggest flaw, which she identifies herself, is that she trusts people. especially given the environment she's living in - she shouldn't, she doesn't want to, but she does it anyway.

i would say that for finn his biggest flaw is his empathy, maybe? it definitely didn't serve him well in the first order and i could see some conflict coming up around how cutthroat the resistance has to be in order to survive.