Yuna's clothing and staff
The Spira's Summoner DLC you'll nab for free with your Amazon pre-order of Lighting Returns: Final Fantasy XIII will be a treat for fans of Final Fantasy X. You'll be able to dress Lightning up in Yuna's summoner dress, equip her staff, and even do her Final Fantasy X victory pose.
Again, this is free with an Amazon pre-order of Lightning Returns: Final Fantasy XIII. The DLC will be available for purchase later this year.
Lightning should also have a Blitzball outfit, I think. And a Wakka one.