TFA burns my ass in a lot of ways upon retrospect, because there's that killer shot of post-massacre Finn which makes the ol' noggin marbles start clackin'.
Imagine an actual exploration of force sensitive Finn whose sensitivity helps in breaking the brainwashing or whatever (he's known already as a problem in the FO based on dialogue, so) and that massacre is the final straw. But this Finn is tired of violence and the brutality of the Order, as well as his own bloody past in their ranks, as much as he desires to break out his FO buddies from their involuntary servitude. So he gets to engage in a Jedi's journey as he finds ways to turn his burgeoning connection to the Force into protection not only for himself but also for all those like him who've been placed into a powerless position. Rather than be an outright coward who's somehow a janitor who got put on an active duty combat mission, Finn is determined to accrue the skills he needs to do some good in his life as he tries to figure out who he is outside of his unwilling role as an FO stooge. (aka what i was super interested in seeing happen that didn't lol)
As a juxtaposition to this, Rey is not the "super kind to everyone she meets" Mary Sue but a scavenger who, as she lives surrounded by old Imperial iconography just as she's obviously come across Rebel things, has a darker connection to the Force. Which she leans into because it helps her survive. She's more likely to do whatever it takes to get paid in food/water and disinterested in anything which threatens what she's carved out for herself in the desert. She's not evil, but she is pragmatic due to her unstable situation.
So you have basically two new-to-the-Force characters with differing views on what the Force is and how it should be used. Their ideals clash as do their desires, so we as the audience get to see both sides of the equation as both Finn and Rey make their choices as defined by their individual experiences/expectations of what the Force can give them and go on divergent paths despite initially following a similar journey once they've met on Jakku.
Plus, due to Finn's newfound weighty character arc that doesn't forget his origins we get insight into the FO so we'd know more things about it than that it exists for some reason. Which could create some neat conundrums for the Rebels Resistance regarding Troopers.
I'm not gonna getting into the nitty gritty with how I pictured b/c it was involved and very autistic. But pared down it would've provided avenues to better smooth out possible Kylo redemption or at least make his background/motivations not stupid, not butcher our beloved OT characters, have an actual Rey-joins-Kylo-and-trains schtick that could've been, y'know, cool, have Rey actually fuckin' fail at things, and even offer a backdrop for a Finn/Rey fight with more emotional weight than the wet blanket shit we got.
Kinda summed up it'd be the selfless (Finn) vs. the selfish (Rey) and get real into how messy white v. black v. grey ideals are.
And Poe is there too I guess.