Susan was lingering in my mind too
I just finished reading "The Last Battle" this evening. I love it. The ending is so joyful and wondrous. I hope that Susan joined them all in the new Narnia, but it seems that she has lost her "faith" or she became too vain. I expected Aslan to to make mention of her, but she isn't a topic after Peter makes his comment concerning her to King Tirian.
I was worried about Susan in "Prince Caspian" because of her somewhat "non-chalant" attitude when she and Peter weren't believing that Lucy saw Aslan. It seemed that she made her heart bit hard more than Peter. Then in Voyage of the Dawn Treader she was hip and happy in New York, and then I thought: "Gee--she is a bit far and away."
If there's any hope; Aslan did say that their parents were there, however their characters were not presented, that is, no one saw them and they didn't speak, so perhaps Susan is there.
Maybe C.S. Lewis left that open for the reader to dwell on. "The Last Battle" does have very explicit Christian teachings, probably the most poignant is that of the dwarves who aren't able to acknowledge the new Narnia and her trapped in their own minds. Emeth has a really great story, worshipping a false god with true heart, and receiving grace from Aslan in the end.