I think my biggest change would be to make Picard's and Kirk's fantasies a lot more appealing, tempting (including making the fantasy wife maybe Beverly or otherwise more compelling and developed, not make the family quite so Victorian-y), and not have echo-Guinan so involved in more explaining the Nexus and helping Picard including meeting Kirk, have Picard learn more about but reject the fantasy and get to know about and meet Kirk on his own.
This is my big problem with the movie too, but I kind of have a polar opposite approach for how it should be looked at
Here's the main problem with the film. It is blatantly obvious that Kirk's fantasy is to command a ship again. Not to be retired. Which was what he was doing before. So why is Kirk's Nexus fantasy having him be retired? And while Picard's fantasy of having a family is debatable, why was he able to escape its allure so easily, despite everything that Guinan said? Or Kirk too, for that matter?
The whole Nexus thing is utterly misrepresented from what they were depicting imho. They claim it's some ultimate fantasy fulfillment, from which no one would ever want to leave, when clearly all it did was take its occupants' current thoughts and extrapolate from them.
They kill off Picard's family earlier,
just so they can make him preoccupied with his family's future, & create a reason for that fantasy. It's not his ultimate fulfillment, which we already know to be captaining a starship. It's what was on his mind at the time. The same is true of Kirk. When he went in, he was feeling put out to pasture, a relic. So, he is literally jumping a horse in a pasture, & realizing it doesn't fulfill him anymore. He was most preoccupied with going & being
home, so he could
tell his woman he was unsatisfied, & needed to go back to Starfleet where he IS fulfilled, & somehow, he's been stuck in a loop of getting that moment... for ages now.
The Nexus isn't an Eden of fantasy fulfillment. It's a mind-reading "Groundhog Day" hellscape, that is probably some telepathic entity's way of keeping its occupants in some form of frozen perpetual contentment, based solely on a cursory scanning of their initial thoughts, likely so it can derive some kind of benefit from that contentment it has rendered in them.
Now consider both Guinan & Soren, who'd been recently ripped from the Nexus, after only a short time of getting caught in it, on an evac from their world, which had been decimated by Borg. Those 2 had lost everything & everyone, like very recently, & the Nexus read them pining for it, in their current thoughts, & gave it to them. OF COURSE
they didn't want to leave.
So there's really nothing wrong with the depiction of the Nexus. It's a perfectly sound Star Trek concept. They just misrepresented it entirely. Its nature needed to be made much clearer to the viewer imho. That & it needed to clean up the timey wimey stuff too