Two problems with that. One, Xion was completely destroyed, all that remains of her are the memories that were in the Journal. So it is impossible for her you physically come back. Two, if Roxas does come back he'll be completely different, as he won't have Ven's heart anymore. He'll look similar to Sora and have no emotions, since he would lack a heart.
but Roxas doesnt have Ventus's heart he shares Sora's it said that in kh:COM that the memories of twilight town are on the other side of his heart which is the Sora's heart that Roxas shares. if roxas was using Ven's heart, Vexen wouldnt of that it was Sora's heart that remembers.
and Roxas doesnt have a heart he shares Sora's and that only gives him access to have emotions.