Currently, I'm on a heavy storyline locations designs: these include Cornelia, Mysidia, the port town, and another port town. In Cornelia, you will be able to start two sidequests: treasure hunting, and Mognet services, and the first one will be needed in order to complete other sidequest(s). There are currently no sidequests for Mysidia and the two port towns, but you can start excavation similar to that from Final Fantasy X-2 in Kaipo.
Dungeons are currectly bit simplistic, but this is mainly for the story dungeons as bonus will be more complicated to complete. I can say that from storyline dungeons there are Chaos Shrine Ruins and Walse Tower, and for optional Antlion's Den.
There will be good bunch of optional and one-time minigames like aforementioned treasure hunting, mognet, desert excavation, but also Cinematic Actions, one of which is in the Media section of the game, and currently there is one one-time minigame, but I cannot reveal it just yet as it would kinda mean spoiling a new location.
As for Final Fantasy characters...You'll find them. ;) Well, some are not entirely "in-character", but they're there.
As for the story: you control a voiceless Warrior and mainly other people conduct a conversation. The beginning of the game will resemble beginning of the first Final Fantasy, so you can expect what to see.