Final Fantasy XIII-2 will reportedly include Lightning from Final Fantasy XIII as DLC. According to Famitsu (via Andriasang), Lightning will be a coliseum character that, once defeated, can become a party member.
Square Enix apparently also stated that it will have "secret story episodes" for Lightning, with more characters from FFXIII potentially having similar story DLC.
The publisher also sent out a trailer covering a variety of gameplay found in Final Fantasy XIII-2, such as puzzles, quizzes and gambling on Chocobo races. Never owe a Moogle money. It always ends badly. Always.