HD re-releases sometimes have the opportunity to fill in gaps for western gamers, and include features or content that never saw the light of day here. Just like Kingdom Hearts 1.5 HD Remix included some content that hadn't made it to our shores, Square will be filling in the North American gaps with the Final Fantasy X/X-2 HD Remaster.
Producer Yoshinori Kitase told Siliconera that the X-2 remaster will be based on the International Version release, which included several bits we never got here. Among them were a "Last Mission" in a new setting, two additional dresspheres including the Psychic and Japanese Festival, and the Creature Creator. That last feature let you capture monsters and surrounding characters, to swap them out as regular party members.