I heard news that the developers were thinking of making a FFXIII-2, that would be great! Though I don't know what story they could pull together it would be interesting, it would obviously mean that Vanille and Fang won't be in it that much. Serah and Dazh should definitely be part of the playable characters!
One thing I truly missed in FFXIII was the ability to explore towns, FFXIII-2 could make this happen, develop on Eden, Palumpolum, Nautilus etc, the glimpses of those towns were fantastic - though they'd probably be abandoned after FFXIII. Have huge towns, where you can actually get lost! lol.
As I think adding anything apocalyptic to the story would just drag it down maybe it should be a bit more light-hearted where FFXIII wasn't at any point. Feature Sazh's chocobo more! Oops I suddenly realised it's Dajh not Dazh - my mistake! Will try to change it :)>
Maybe they should have them doing some sort of treasure hunting with some mysterious subplot- you can see I'm not born to be a game designer! lol.