There are no spin offs in the KH verse. Side stories, maybe, as Sora isn't the protagonist, but no spin offs.
A spin off would be to give Kairi a Keyblade and let her off on her own adventure, or have Riku and Selphie's children wonder off to save the world with Donald's nephews.
There's actually basis to at least mention her, seeing how Much like Namine, despite Riku and Roxas forgetting her, the Memories are still there in their Hearts (well, in Sora's Heart in Roxas's case). Not to mention that Xion is now a part of Sora, and seem to be related to the Oblivion, going by how Riku heard her voice when Roxas threw it at him. And how she talked at the end of the game to Roxas, despite having disappeared. And of course with the BBS implications she has, she's likely to be mentioned.