It's been documented that Shatner considered quitting Star Trek Generations when he received the script and found that Kirk didn't really have an important role in it, and seemed to be "spending the entire movie waiting around to get killed off". That's why later rewrites added in Antonia and the horseback ride scene, to give Kirk more of a character arc. On the face of it, it looks like they would've been screwed if Shatner had pulled out since the entire crossover depended on his involvement – but I think I've come up with a way they could've rewritten Generations as a purely TNG film, and possibly ended up with a better result.
To begin with, the opening sequence can still have the El-Aurian refugees on the Lakul being rescued by the Enterprise-B – but it's told from Guinan's point of view. We begin the movie with the Lakul running into the energy ribbon; they drop out of warp, they're getting drawn further in, structural integrity failing, the captain of the Lakul sends out a distress signal, lights fail and consoles explode, etc... then a heroic fanfare as the Enterprise-B swoops into view for the first time! This is not the Ent-B's maiden voyage: they've been out in space for a couple of years and there's no indication they're unprepared. Their transporter operator is having trouble getting a lock: the lifesigns on the Lakul keep fading in and out for some reason. Back on the Lakul, amid the smoke and the emergency lighting we can just see that the people seem to be fading in and out of existence – finally they are beamed away just as the ship explodes. Now in the Ent-B's sickbay, the refugees are crying and panicking, and Soran starts screaming that he has to go back before a nurse sedates him. Guinan seems like she's in shock; one of the medical staff shepherds her over to a bio-bed and asks what her name is – she answers "Guinan" before passing out. The Enterprise-B breaks free of the energy ribbon and out into space. Dissolve to Worf's promotion ceremony on the holodeck, with the caption "75 Years Later" (to allow for the Enterprise-B to have been in space for a while already).
Now, there's quite a few things I'd change in the movie (I wouldn't kill off Rene, I would use the idea introduced in "Descent Part II" that the emotion chip contains memories, I would replace the Duras Sisters with Sela gone rogue, I would leave in the scene where Soran tortures Geordi, and I would make Picard's Nexus fantasy very different) but right now I'll just talk about what I'd do to replace Kirk. First of all, I'd clarify why Guinan has an "echo" in the Nexus: it's because she was unnaturally "ripped away" when the Enterprise-B beamed her away, which essentially tore her soul in two and left a piece of her behind in the Nexus. And I would say that Echo Guinan can leave the Nexus – and if she does, she will reintegrate with the real-world Guinan who is aboard the Enterprise-D. So Picard and Guinan willingly leave the Nexus together, and they stop Soran together. There are a lot of suggestions throughout TNG that Guinan is actually really badass (she's a better shot than Worf, she keeps that huge gun behind the bar, she even intimidates Q!) and now we get to actually see her in action.
Do you think that could've worked?
To begin with, the opening sequence can still have the El-Aurian refugees on the Lakul being rescued by the Enterprise-B – but it's told from Guinan's point of view. We begin the movie with the Lakul running into the energy ribbon; they drop out of warp, they're getting drawn further in, structural integrity failing, the captain of the Lakul sends out a distress signal, lights fail and consoles explode, etc... then a heroic fanfare as the Enterprise-B swoops into view for the first time! This is not the Ent-B's maiden voyage: they've been out in space for a couple of years and there's no indication they're unprepared. Their transporter operator is having trouble getting a lock: the lifesigns on the Lakul keep fading in and out for some reason. Back on the Lakul, amid the smoke and the emergency lighting we can just see that the people seem to be fading in and out of existence – finally they are beamed away just as the ship explodes. Now in the Ent-B's sickbay, the refugees are crying and panicking, and Soran starts screaming that he has to go back before a nurse sedates him. Guinan seems like she's in shock; one of the medical staff shepherds her over to a bio-bed and asks what her name is – she answers "Guinan" before passing out. The Enterprise-B breaks free of the energy ribbon and out into space. Dissolve to Worf's promotion ceremony on the holodeck, with the caption "75 Years Later" (to allow for the Enterprise-B to have been in space for a while already).
Now, there's quite a few things I'd change in the movie (I wouldn't kill off Rene, I would use the idea introduced in "Descent Part II" that the emotion chip contains memories, I would replace the Duras Sisters with Sela gone rogue, I would leave in the scene where Soran tortures Geordi, and I would make Picard's Nexus fantasy very different) but right now I'll just talk about what I'd do to replace Kirk. First of all, I'd clarify why Guinan has an "echo" in the Nexus: it's because she was unnaturally "ripped away" when the Enterprise-B beamed her away, which essentially tore her soul in two and left a piece of her behind in the Nexus. And I would say that Echo Guinan can leave the Nexus – and if she does, she will reintegrate with the real-world Guinan who is aboard the Enterprise-D. So Picard and Guinan willingly leave the Nexus together, and they stop Soran together. There are a lot of suggestions throughout TNG that Guinan is actually really badass (she's a better shot than Worf, she keeps that huge gun behind the bar, she even intimidates Q!) and now we get to actually see her in action.
Do you think that could've worked?