I imagine it will be mostly cosmetic, like the ability to wear all the Advent Children outfits or maybe ones from KH or Amano. If there is story DLC, I feel the smart move would be to make it original stuff. Like how about a story mission where you play as Biggs, Jessie, and Wedge? Or maybe you can play the Yuffie Wutai mission from the Turks perspective?

I don't imagine anything from the main game fans know will be cut and used for DLC, that's just begging to have fans turn on them.

As for optional summons. I'd imagine Diabolos and Fenrir will make it into the game as either DLC or just new summons in general. I also wouldn't be surprised if we get a new Bahamut as well. Maybe his CC or AC incarnations if its DLC.