Adding DLC on Final Fantasy XIII (Xbox 360, PlayStation 3) is not the only thing that Square Enix is taking into consideration. Yoshi Kitase reveals that they are in fact considering putting downloadable content to future Final Fantasy titles.
At the GamesCom event, Kitase explained (via translator) that:
As far as DLC is concerned, there is no solid plan at the moment for FFXIII. However, we've been listening to overseas fans in Europe and America and are aware that DLC is very popular and something that people really want. So it's definitely being considered for future projects.
Yes, listening is good, so do listen well to your fans, please, Square Enix. If you really want to prove that you do listen to what your loyal followers are saying, you can start by stop considering and actually put DLC on FFXIII.