So far it's a bunch of boss battles
It was recently revealed that Bravely Default would be getting microtransactions, but you can add DLC to that pile as well. Although some boss DLC has already been revealed in Japan, it appears as if even more downloadable content is on the way for the game -- including Final Fantasy: The Four Heroes of Light crossovers, because, according to producer Tomoya Asano, "parts of the artwork are similar."
The way you'll interact with them is by way of Wi-Fi connectivity, through a StreetPass centric town built into the game. You'll also be able to pick them up like a virus from other 3DS owners who have encountered them. According to Asano, there are plans to add even more bosses to the game, which is great news if it's all free DLC.