In all honesty, if Xion ever returned, I'd be quite upset. I already hated her plotline enough as is; if she returned, it would ruin the one single thing I liked about it -- her willingness to accept her fate for a higher purpose. Let her character mean something through her sacrifice; it's the thing that makes her unique. Bringing her back so she can do what we already have 7+ characters lined up to do (eat ice cream, slap stuff with keyblade) is patently useless.
I feel the same way about Roxas and Namine, though since they've been two of my favorite characters for years. I think it's best that some characters just stay with the ending they were given, so 1) we don't ruin their significance, 2) we can get rid of as many Sora/Kairi/Ventus clones as possible, and 3) we can make room for more characters in the future.