I liked the movie a lot. TL;DR the only major change I would have liked was Han living and Leia being killed by her son.
While I understand why Han had to die I think killing of Leia would have been better. First reason is obvious: it was painful seeing Fisher on screen whereas Ford carried his role well.
Second, is as a plot vehicle Ren killing Solo works but Ren killing a woman, his mother no less, would be a traumatic and dramatic turn to the dark side. Such a turn would have focused Han (become the general of the Rebels he was destined to be?) and Luke would obviously feel such a disturbance in the Force and "Awaken" him. Kylo Ren would be on a path deeper into the Darkside than those before him. Yes, the previous Sith Lords killed/wanted to kill their masters, killed Jedi's, and killed innocent people. Kylo Ren was situation in Ep. VII to:
Exile his master (Luke)
Kill a school of young Jedi
Murder his own mother
The Dark Side would have been complete in him. All that would be left, after being wounded like his grandfather, would be to embrace Darth Vader and become Darth Vader II--stronger than ever and beyond being turned by the love of his uncle (Luke) as he had embraced the deepest of treachery: betraying his father by killing his own mother only to draw strength from seeing his own father (Han Solo) grow in pain, hate, anger, and disdain for his own son. Oh, and Ben Solo would have been of Skywalker blood which means the force would naturally be extremely strong in him.
The Leia killing Ben Solo would have created a much more sinister, and powerful, Sith Lord. And whereas seeing Kylo Ren try to barter with his estranged father (who wasn't always a great father) it would have been beyond powerful to see Leia, a force sensitive figure, believe she has returned her son to the Light Side of the Force only for him to at the most inopportune moment run her through face-to-face. I can only imagine Kyle Ren, verbalizing his deep and sincere love for his mother and yet coldly explaining her death was to complete the circle and to bring Han and Luke down to their end and that Kylo could rightfully take up the place of his grandfather Darth Vader as Lord of the Sith.
I shutter at the thought of the Luke/Leia re-union. Leia's death would have painted the most bleak of futures; Han's paints a possible path to the Emo Side of the Force.
Third, is a big part of the original trilogy was Luke and Han being pulled together and being complete opposites. (Yes, Han/Leia also had a big part in the chemistry but that is all but gone.) With the turn of Ben Solo to Kylo Ren with the mother, Leia, out of the way you could see the natural need for Luke and Han to team up yet again but with the obvious tension: Han/Leia could not handle Ben, but under Luke's supervision he completed his fall to the dark side. And neither could protect Leia and would blame the other for her death.
Forth, while I didn't have an issue with Rey's force abilities I think it would have been more dramatic for Leia's death drawing Luke out of his depressed, isolated exile by the sheer pain he would feel in the Force. Seeing some snippets earlier in the movie of Luke (e.g. putting a memory block on Rey as the last of his apprentices and the lone survivor of his academy; a bitter dispute with Han/Leia regarding Ben Solo and Luke imposing a self imposed exile due to the dangers of the Force and Skywalker clan) and finally a "Force Vision" where Leia with Obi-wan and Yoda at her side, calling out to him saying, "Luke, you must return. You are the last of the Jedi. Without you there is no hope."--and after the vision disappears being transported to the "Jedi heaven" where Yoda tells Leia, "Another Jedi, there is" only for Obi-wan to interrupt, "Master, do not underestimate the power of the Force in Luke. He must return balance to the Force. Until Luke completes his path as Jedi Teacher his apprentice will fail."
This would have setup a lot entry by Luke to save Finn and Rey and for it to be Luke who inflicts the mortal injuries that will turn Kylo Ren into Darth Vader II. This would have completed the circle. Having Luke regret sparing Ben Solo after he killed his apprentices would have been a deep wound, and then his sister would have brought much anger from Luke. (And maybe even be the motivation of Emperor Snook--to turn Luke.)
It could also create the tiers of Jedi powers: We see Ren destroy Finn, Rey give a good fight only to be bested by a near-Sith Lord apprentice, and then for Luke to enter as pure Jedi being reminded of his progression in Ep. IV-V-VI in how in V with minimal training he showed promised by Vader destroyed him but in VI he was able to destroy Vader. Seeing Luke "go easy on Ren" while he tries to reason with him to have 1 last chance to turn from his path and when all is lost and Ren makes his move (whatever plot device--e.g. he lies and says he killed Rey, "I didn't make the same mistake I did last time when I spared Rey... your daughter.") Luke says "screw it" and breaks out the full wrath of his force powers that we have never seen before. Watching Ken be injured in ways to turn him into Vader and for the battle to end when Rey appears on scene to temper Luke's anger and her pleas for him to leave the planet before it explodes and Luke leaving Kylo Ren to die just as Obi-wan left Annikan.
That sounds like a lot, but basically 5 minutes of Luke screen time (most at the end) and then swapping out Leia for Han for future movies.
Again, I really like the movie and I understand the new direction/new actors. But I would have preferred the above fall out from Luke/Han than with Leia.