I don't think that is going to happen. Anything FFXIV could say about the multiverse as a whole can have no bearing on earlier or later games (like FFIX's mothercrystal, it would still have to let the other games keep their own crossover rules), so I'd honestly rather they didn't try at all to explain the deliberately unexplained. There's already no one rule that could possibly explain everything... unless they go the Dissidia route and have all the "crossover" characters pulled from some specific MacGuffin that doesn't have any actual connection to the games in question.

Say whatever you want about Nomura and his love of crossovers, he's always been very careful not to try and set anything in stone about the 'verse. It's better that way, I think.