Well, technically there are optional storylines post main scenario you can do - but these are just the quest lines and associated storylines connected to the Crystal Tower and Binding Coil of Bahamut raids (a raid is like a dungeon shawn of exploration and basically one long set of battles, one after the other, with bosses every so often).

Apart from Labyrinth of the Ancients (the first raid of the Crystal Tower), these require a high average item level, and at the very least, your relic weapon to undertake. They're technically part of the main story, but they're entirely optional and do not have to be undertaken if you do not wish to do so.