Final Fantasy XIV is already a hodge-podge of Final Fantasy royalty, thanks to cameos from elements like Magitek Armor from FF6. Square Enix sees other cross-promotional opportunities to toy with, like having Lightning from the upcoming Lightning Returns: Final Fantasy 13 appear in special events.
A new video shows Lightning appearing in one of FF14's Full Active Time Events (FATE). You can fight alongside her, and completing the event will grant you some gear and weapons modeled after Lightning Returns.
In addition, Square released some screenshots of Lightning in her own game, wearing a costume modeled after the Miqo'te, a cat-like race found in Final Fantasy XIV. The company says you can find clues on how to acquire the costume scattered around the world, but you could just as easily wait for someone else to find the way and put it on the Internet. You'll also be able to do side-quests in exchange for weapons and shields modeled after the three city-states in Final Fantasy 14.