Yoshida's has already stated in the past that the 24-man raid will not be taken from a previous FF game, and that he doesn't want to establish a precedent for "nostalgia raids" as the only 24-man content FFXIV has. But now we have this.

I'm thinking that this may be our next Praetorium, or at the very least our next Mor Dhona. But I have no idea how, yet, and by now it's safe to say that any time they drop speculation-bait like this the real answer is always something totally outside the information they've given us beforehand.