As of now, we only know the following:

- Eureka; supposedly an open world-esque area separate from current zones but not another Diadem.
- Something "brand new" for the genre.
- PotD; Yoshida specifically mentioned the preference people had for the higher floors
- Blitzball (siiiiiiiigh)
- A single "Brutal" difficult encounter said to be "nostalgic." Community guesses are Twintania, Bahamut or Ozma

Could be plenty more, but these are the only things brought up in live letters or Q&As. There has been mentions of a glamour history/wardrobe of some sort. Pray for it.