I'm starting to wonder if there's a a way for a shard to be rejoined without a calamity being caused, because those cryptic words in the 4.4 trailer feel like they're implying that the first shard might be beyond saving and becoming a threat to the source somehow. Considering the ongoing problems the XIVth has been causing after becoming a void of darkness and breeding hostile voidsent who constantly seek to invade the source, I'd think that a shard that becomes a void of light could cause problems of its own.

"Throwing wide the gates" really sounds like it could be referring to removing the barrier between the first and the Source to cause a rejoining, perhaps to "save" what souls remain in that world by allowing them to return to the lifestream instead of being trapped in a state between life and death and also to avert a calamity from an overflow of light bleeding into the source.