You would think Roxas’ and Xion’s arcs were finished. Xion disappeared into Roxas, and Roxas had to sacrifice his consciousness for Sora. But the KH3 box art says otherwise, and it’s not too difficult to see why.
Many of the remaining plot threads revolve around sorting out the messes made of various characters’ hearts. This, of course, includes dealing with the Russian doll-type scenario of having more than one personality in a given body.
Roxas is too important a character to leave “disappeared,” especially since he ties in with Ventus. However, if something happens with Roxas outside of Sora, then it follows that something will happen with Xion, the littlest doll in this set.
From an emotional satisfaction perspective, it would be nice to see each of them develop their own, distinct personalities and live happy lives from there on.
Yet it seems there might be some character resolution between Roxas and Sora necessary for either of them to move on. If Nobodies are the dark side of a person, and Roxas is Sora’s Nobody, then there’s some darkness in Sora that hasn’t yet been dealt with.
Given Roxas’ chief desire at the beginning of KH2, it would probably be something to do with Sora as reluctant hero. If Sora satisfactorily deals with this, maybe Roxas can be put to rest like Axel, though what it means for Xion is anyone’s guess.