Yoshida has stated at least once that once the Crystal Tower events are wrapped up, there will be another "throwback" set of dungeons/raids directly inspired by another Final Fantasy game.

Personally, V is my favourite Final Fantasy game (that job system is so good), so I'd be all for it. My personal choice would be a series of raids in the Cleft of Dimensions, probably fluffed in XIV as a civilization that, in communion with the void, accidentally got its entire island swallowed up. Many of V's most recognisable bosses appeared in the Cleft (Melusine, Azulmagia, Twintania, Halicarnassus, Necrophobe, Shinryuu, Omega...) and just think of the visual effects they could apply.