Well.. the more missions I do the more backstory you can read in the datalog. They are actively trying to create a backstory for the world of Pulse. Either because they already are planning on creating another game in the exact world (Versus/Agito) or they are just plain setting themselves up for some sort of prequel/sequel. I would just guess that Versus is going to be set on Pulse a thousand or more years before 13. Maybe show how Cocoon was built and became enemies of the Pulsian people/Fal'Cie.
A DLC would be decent, but honestly, a lot of people wouldn't pay for a comprehensive "mission" DLC for 13. The story is what they are there for. I am enjoying doing the missions, but at some point I will just say the hell with it and move on to a new game. If a DLC involved finding which Fal'Cie originally marked Vanille and Fang (we are led to believe its Anima but it may not be) and defeating them in order to uncrystalize them, that would be an interesting "mini story" that would be infinitely fun.
qft
--
Just wanted to go back to the talk on what happened during the "fal'cie smoke and mirrors, party turns to l'Cie" portion of the ending. The more you go back and actually pay attention to that CS, the more it is clear to me that the reason why Fang's mark is white is because the Maker interceded and stopped her the first time and then the Maker interceded with the group to bring them back thus making their mark's white as well. They speak of someone "showing them the way" and "light" and such. ie its an allusion to God helping them out (yes in JP version they just plain called it God).