Final Fantasy XIII-2 focuses on Serah's search for her missing sister, Lightning. The lead character of the first game plays a smaller role in the sequel, but DLC will reportedly allow players to recruit her. The add-on content will potentially add story revelations as well.
The latest issue of Famitsu (via Andriasang) reports that Lightning is set to appear in the Coliseum. If you can take her down, she'll join as a party member. Square Enix also reportedly plans to release story episodes for Lightning. This is part of a plan to expand the plot in a variety of ways, including novelizations.
Other possible DLC or novels include Coliseum battles against classic Final Fantasy bosses, and stories revolving around Sazh and his son, and the shop owner Chocolina. This echoes earlier hints regarding Sazh appearing in post-release content.
Because Final Fantasy XIII-2 has already released in Japan, it's no surprise that Square Enix is already announcing its DLC plans for the game. While no announcements have been made for the US, it seems likely that this content will come to our shores eventually.
Final Fantasy XIII-2 will be available on PS3 and Xbox 360 on January 31st.