The first time I read the Last Battle, I was surprised by Susan’s fate, too. She seemed like a really sweet person, and I liked the practical streak that she had. I also related to her sense of fear in tense or new situations, since a lot of books don’t have people being scared during adventures, but I think that a fair number of people in reality would be, even if they had to overcome those fears in order to succeed on their quest.
In LWW, Susan the Gentle would seem like such a perfect candidate for admittance into Aslan’s country. I mean, she was one of those Aslan chose to be Queen of Narnia, and she was also one of the two people He chose to accompany Him on His journey to sacrifice Himself on the Stone Table. That’s comparable to being one of those at the foot of the Cross when Jesus was Crucified, and, Susan being with Aslan when He rose from the dead also means that she was present at the Narnian equivalent to Jesus’s resurrection in the tomb. Of all people, then, we would expect Susan to understand how powerful Aslan is, and how He is capable of saving everyone, and how He can bring triumph out of tragedy. Yet, in LB, we see that she doesn’t even believe in Narnia or Aslan. I guess that can be a sobering reality check for people who sometimes say or think things like, “If I could have been there when Jesus made the blind see or rose from the grave, I would believe,” because Susan was there for Aslan’s resurrection, and she didn’t have faith. Clearly, nothing—no miracle—can be strong enough to make someone determined to doubt believe.
I guess that brings us to the question of whether Susan was determined to doubt. I think that events in PC and the comments of other characters in LB support the argument that, at least as of the end of the Narnia series, Susan was determined to doubt. In PC, we see that Susan, by her own admittance, thought that she saw Aslan, but she didn’t want to follow Him because she was afraid. Contrast this with Peter, who, while he was wrong not to listen to Lucy when she claimed that she had seen Aslan, genuinely did not see Aslan and probably would not have been afraid to follow Aslan if he had seen Aslan. Peter’s problem was that he didn’t see Aslan; Susan’s was that she saw Him and then tried to convince herself, out of fear of what following Him would mean, that she hadn’t seen Him. That indicates a person who is willing to give into her fears and create doubts in her own mind even though, in the past, her experiences demonstrate that she has ample reason to believe.
I also suspect that events at the end of PC might reinforce the separation that she was beginning to feel toward Aslan during her second trip to Narnia. Susan might have felt betrayed and hurt when she was told that she wouldn’t be able to return to Narnia, so, as a defensive mechanism to, in her mind, spare herself further pain, she might have decided to embrace the more superficial elements of this world (nylons, lipstick, etc.) instead of sitting around with her siblings and rehashing old Narnia tales. To me, we see hints of her orientation with the more superficial aspects of this world coming out at the end of PC. When she and Peter explain to Lucy and Edmund that they won’t be returning, it is Peter who tells the younger ones that it’s all right they (Susan and Peter) won’t be coming back to Narnia and that they (Edmund and Lucy) will understand that when it is time for them to leave Narnia. On the other hand, Susan’s concern is of a more practical nature—she just tells the siblings where they can find their school clothes to change into so that they won’t look out of place at the train station they’ll be returning to. From that, we really can’t make a definitive statement about how well Susan has accepted not being able to return to Narnia. However, based on the descriptions of her behavior in LB, I feel that she might have had difficulty dealing with the fact that she wouldn’t be able to return to Narnia, and she might have dealt with that by focusing only on very practical, worldly matters.
Here I should probably say that Susan’s mistake, in my opinion, wasn’t her wanting to grow closer to Earth, per say, because, if Aslan’s instructions to her and Peter were at all reminiscent of those He offers to Edmund and Lucy at the conclusion of VotD, He might very well have advised Susan and Peter, “You must begin to grow closer to things of your own world.” Her mistake, I think, is in wanting to grow close to the wrong things of this world. She doesn’t want to engage in meaningful relationships with people on Earth. She doesn’t want to volunteer or find a job that helps other people. She doesn’t want to go out and enjoy nature on the weekends. All she wants to, by the sound of it in LB, is make herself look fashionable and behave in a way that will allow her to be popular. Aslan would have wanted her to live in the world but not be of the world, but based on the account of her conduct in LB, she seems to have chosen to be of the world and not just to live in it. She also chose the things of this world that would draw her farther away from Aslan, rather than closer to Him. That is her error, and I think it stems from being willing to listen to the fears and doubts that we see beginning to chip away at her in PC.
As to her fate after LB, I don’t think that she will get to the “New Narnia,” because by the end of LB, it seemed like all the inhabitants of the “New Narnia” were moving farther in and farther up to a heaven where all the new versions of all the worlds seemed to come together. I think that was the country Aslan was referring to in VotD when He told Lucy and Edmund that there were ways into His country from all worlds. I think that when a world ends and all the righteous of that world join the new version of that world, all the people in that new version are lead by their world’s Jesus up to that heaven where all the righteous people from all the new worlds departed live in eternal bliss. I do feel that it is possible that Susan could find her way to that country through Jesus, but I think that she would spend time with her parents in the New Earth on the way, and then she would, when our old Earth ended, join her siblings in the paradise where the righteous of all worlds are united. To us, that may seem like a long time for the Pevensies to wait for reunion, but I think, to those outside the constraints of time as we know it, it could feel like no more than an eyeblink.
I think that the deaths of her parents and siblings provide Susan an opportunity to look at her life, repent for her sins, and seek to reform herself. Her grief has the potential to drive her to Jesus as a source of hope and salvation. However, there is also the equal and opposite possibility that her loss will turn her against God. The feelings of hurt, betrayal, and resentment that she may have developed at the end of PC as a result of being told that she wouldn’t be returning to Narnia might intensify. Again, Susan has the chance to choose whether she will listen to her fears and doubts, or whether she will overcome them through faith. That, I feel, is why Lewis decided to leave her fate open-ended at the conclusion of the series.
So, yes, there is hope for Susan, but there is also the danger of what might happen to her if she does not repent and reform herself.
As for what I would do if I were writing a story about what happened to Susan, I would probably want to put her back onto the path of salvation. I would like to give her friends and a boyfriend who were actually a lot more loving and accepting of who she was as a person (not of the mask she pretended to be with her lipstick and nylons) than she would have thought imaginable. I would have her breaking down in her grief before them, and feeling like she was going to lose all her popularity as a result, only to discover that her friends and boyfriend are happy to have more of a glimpse of the real her and want her to show that side of her more, because they like it more. I would like for Susan the Gentle to develop meaningful, rather than superficial relationships, with the people of this world. I think she would make a great mother and wife, and that she could, if she wanted, do a wonderful job organizing charity events. She could put her practical instincts to brilliant use furthering God’s kingdom in such ways. It is entirely possible that God had more work for her to do on Earth, and that’s why she didn’t die with her siblings.