Well SE could not be bothered adding side missions and town exploration to FFXIII to make the game more enjoyable and extend the game.
Renting or buying FFXIII to me personally would be too much hard work. Considering the sub par JRPGs Square Enix have released this generation I do not want to buy any of their games.
Maybe the DLC will be extra towns that you have to pay for. Content excluded by developers from FFXIII could of been included in the game.