World of FF is a Action Role-Playing Game releasing on the PS4 and PS Vita later this year from Square Enix, with a major goal of the project being to introduce the FF series  new generations and celebrate the series with cameo’s and appearances of famous characters. Its announced characters like Cloud (FFVII) and Lighting (FFXIII) will be appearing in the game but others were announced to appear in the game today including Undead Princess from Crystal Chronicles and Odin from other FF games.
Sources: NeoGaf & 4Gamer