Okay, this is a complaint that applies to the sequel trilogy overall, but I figured I'd mention it here since it's a big part of TLJ.
This thought just randomly struck me the other day...why does Han Solo have a son in these movies?
I know the idea of Han having kids goes back to the expanded universe...but is it really necessary? I mean, did Han Solo ever strike anyone as a "family man" kind of guy? I never really saw Han Solo as the kind of guy who'd want to settle down and raise kids. Heck, I never even saw him as the kind of guy who'd want a long-term relationship. He's a loner and a scoundrel, and TFA goes out of it's way to show us that Han hasn't changed much. If the old EU hadn't given Han kids, I think this decision would be seen as kind of out-of-place.
What's makes it weirder is that they didn't give Luke kids. Out of the two, doesn't Luke seem more likely to raise children? I know people change over time so I can't necessarily call either one "out-of-character", but this decision seems kind of arbitrary and even counter-intuitive.
What TFA established was all perfectly in character for the OT Big Three.
Han and Leia were obviously going to have kids. By the RotJ, he'd gone from rogue scoundrel guy (who SAYS he doesn't need anybody or care about anything, but his actions constantly prove otherwise, from as early as chasing away that squad of Storm Troopers on the Death Star) to full blown responsible in-it-for-the-long-haul leader.
That said, while people do change, they only change so much. After RoTJ, it stands to reason that Han and Leia-- while loving-- wouldn't necessarily be the best parents. Leia's a workaholic who's wiping up the remnants of the old Empire and helping build a new galactic government, and Han's a general who's not emotionally equipped to deal with a potent Force User who's "got too much Vader in him". While I'm sure he and Ben had things in common (hot-headed pilots), the Force stuff is quite a gulf between them. And while Leia has all that power, she doesn't train or really think about it-- it's just not her thing.
When Ben finally, officially turned, it makes sense that Leia would bury herself in her work and Han would backslide into his "young carefree" self so he can hide from the pain. It also consistent that he wouldn't be able to help himself from helping others when the time came, and from trying to win his son back. It's sort of the perfect ending for him.
Luke's a loving guy, but over the course of the OT, he becomes more and more detached. He's emotional, sure, but not in the same way as the others. He processes things differently because of his growing connection the Force, as well as often being physically absent from the rest to go pursue his Jedi stuff. It's his compassion that saves Vader, but it's Force driven rather than anything truly familial (since, y'know, they didn't really know each other beyond fighting and sensing each other sometimes). It stands to reason he'd continue all that post-RotJ. I also think it's telling that Leia says "Luke's a JEDI, you're his father" rather than "Luke's his UNCLE" in TFA. Hell, even in TLJ, Luke's saying things like "a boy whose MASTER had failed him" rather than uncle. Luke was never going to have kids, he was all about exploring the Force and the Jedi.
TFA was incredibly well thought-out and emotionally honest when it comes to those guys. But, that's Abrams. The mystery box and nostalgia are things for him definitely, but at the core, the guy loves characters and arcs. Johnson, to me at least, seems to be about the THEME of the overall movie, then manipulating the characters to fit that where he needs to. Both ways work great, but I don't think Johnson pulled it off (much at all really) this time.