1. I didn't like Mary Poppins Leia at all, BUT...I think I would have been more okay with it if someone had acknowledged how crazy it was at some point. The crowd can't be the only ones who are thinking WTF just happened?!? The fact that it happens and is completely ignored makes it that much more unbelievable to me. At least have someone like Poe be like "what the hell was that?" Or a short scene with Leia after she's woken up when someone asks her what happened and she's like "I don't know, I just knew that was what I had to do."
Honestly, I wasn't bothered by "Mary Poppins Leia" as others were, mainly because the Original Trilogy established that, being Luke's twin sister, meant she's just as potential powerful in use of the Force as he, and after thirty years, she more than likely learned a few things or two. But I do agree, it would've been nice if Poe, Finn, and others are flabbergasted that she's a Force user.
3. Admiral Ackbar should have survived the bridge bombing and been the one to fly the Raddus into the Star Destroyer. What better homage to Admiral Raddus than to have Ackbar save the day on the ship named after his kin. Also, I totally would've been okay if one of the First Order officers called out "It's a trap!" right before it happened! That's a joke I'd get behind 100%.
Yes! In fact, it would've made much more sense to give Larua Dern's character's story to Admiral Ackbar.
EDIT: Or, if you really want to give fans a punch in the gut, have it be Lando Calrissian.
Anyways, here's the things would've done differently off the top of my head.
1. Instead of having Snoke be some Sith Lord we've never heard of or be introduced to before, I would've kept his face hidden in The Force Awakens, and then reveal in this film that he's...Darth Maul. Yes, I know what ultimately happens to him in Rebels, but I'd change that too. Besides, it could actually fit with what the themes of this new trilogy apparently are. Here you would have a character, from the Prequels no less, who not only is trying to rebuilt the Empire his former master started, but believes he can do a better job. Not to mention he'd be so old, twisted, and broken that his power comes from leeching off other Force users like Kylo Ren. It would also fit perfectly with the Johnson's use of the color red in the film.
2. On a similar note, have it so that the Praetorian Guard are the Knights of Ren.
3. One of the biggest criticisms is that if Poe was let in on the plan from the start, he wouldn't have done what he did. Well here's an easy fix: have it be that he's told exactly what Leia and Holdo's plan is from the start...
and he still thinks it's stupid. Why? Because even if they make as far as Crait, it doesn't change the fact that the fleet can't jump into lightspeed without being tracked, that they're running out of fuel, and that they're slowly being picked off one by one. At least disabling the First Order's ability to track them would save more lives, he reasons. So when he actually commits mutiny, and tries explaining his actions, he gets thrown in the brig.
This also would give greater weight to the "hyperspace scene" because, through Poe's good but reckless intentions, he indirectly made someone else sacrifice their life to save them. And make his run on Crait be his one last shot at redeeming himself.
4. When Luke "appears" on Crait, make it so that, other than shorter hair, he still looks as old as he does on Acho-To. Also, make his lightsaber green. And, to top it off, have him actually parry Kylo Ren's saber strokes along with being able to dodge and weave them. After all, even though Luke is a Force Projection, he's still solid like when he kisses Leia on the forehead, etc.
Those are the things which I think of off the top of my head at the moment.