No one knows if SE will ever bring back the FFXIII event (or the FFXI and Dragon Quest events too for that matter). They did run them again mid last year as part of the celebration for ARR"s first anniversary of service, so they might decide to do something similar this year to celebrate Heavensward's release. But that's a long shot.

Otherwise, it's also possibly it might be offered eventually for purchase on the Mogstation, but that's just speculation on my part too.