Well, the Two become one is themed after Roxas, so it would not be that farfetched if this one becomes his default Keyblade when his own heart is awakened.
In the last part of the DDD opening though he is again seen wielding both Oathkeeper and Oblivion, and these seem to be more or less staples for him as well.
I also wouldn't mind if for some reason Roxas retains his dual wield ability and becomes the only one to be able to use it constantly. It would add more variety to the different Keyblade wielders (like Ven's backhand-wielding) and give a strong incentive to get him as a possible party member.
As for Xion, her Keyblade was fake and she could only wield by sapping Roxas' powers.
I am all for her to come back, but maybe she should no longer be able to wield a Keyblade. Give her a mage staff or something. ;P
