You can make it work.
* Eight years after The Last Battle, Susan Pevensie has grown up on Earth, married, and have a child (Lucinda Eddie-Paige).
* The loss of her brothers, sister and other friends have made Susan meditate in her life, priorities, beliefs and is now a strong beliver in Christ (which revealed Himself to her as Aslan when she was a young teenager).
What you need to set well in your story is how to reconnect Susan to Narnia, I mean:
* The old Narnia world / realm that Susan ruled in / traveled to (refer to The Lion, The Witch, and the Wardrobe; Prince Caspian, and The Horse and His Boy) is destroyed at The Last Battle. Susan can not travel back to "old" Narnia like in the CS Lewis novels.
* The "new" Narnia or the heaven Narnia where Susan's brothers and sister went at the end of the Last Battle is equivalent to heaven, so to get there Susan and Lucinda would have to die.
Option 1: Travel to New Narnia
* Susan and Lucinda have an accident and fall into a comma, clinically dying for a few moments but the doctors where able to bring back them to life here on Earth.
* During that short time they experience a long adventure at "new" Narnia (remember that time in Narnia runs at a different rate than time on Earth). Those few seconds "dead" become weeks or months in "new" Narnia.
* However, "new" Narnia is Heavens, there is no evil in Heavens, there would be no evil witch, or evil ruler to fight against in "new" Narnia.
* The story would be Susan meeting again with Peter, Edmund and Lucy; and Lucinda getting to know her aunt and uncles. They tell old adventures to Lucinda and we to read what other events ocurred in "old" Narnia during the years the four Pevensies where Kings and Queens of Narnia, similar to The Horse and His Boy.
Option 2:
CS Lewis did not included traveling to the past in his Chronicles of Narnia novels, that would be something new from you.
* Susan and Lucinda find a way back to old Narnia arriving in a unknown time in the past, probably:
*** After the events of "Silver Chair" but
*** before the events of "The Last Battle".
The story would present Narnia ruled by a grand son or great grandson of King Caspian X who needs help to deal with a big problem. He is not Prince Tirian (the protagonist of The Last Battle), Tirian is not born yet. Therefore the destruction of old Narnia narrated in The Last Battle is still in the Narnia's future (but in Susan's past).
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I know strictly narnia is destroyed but there HAS to be something left.
* I consider the islands, the sea, Archenland, and lands beyond there also destroyed after the events of The Last Battle. Narnia was a world, a complete planet, not just a continent. every land or place at "old" Narnia was destroyed.
* The "Wood between the Worlds" is another location, not part of the Narnia "planet"; therefore it is still there. However, the pool (or pond) that connected from the Woods to Narnia is dry now, since "old" Narnia was destroyed.
* If Susan and Lucinda get to the "Wood between the Worlds" they could visit another world, (like when Digory Kirke and Polly Plummer visited Charn), but they can not get into Narnia (old Narnia) anymore (unless traveling back in time).
Maybe a few Narnians were somehow left behind in another world and it is up to Susan and Lucinda to find them going through the "Wood between the Worlds", but how to lead them to New Narnia? Leave that to Aslan.