Based upon on the information I have relating to FF14 Fanfest, here is what I consider the plot for FF14 Shadowbringers.

1. A Ascian/something corrupts Hydaelyn.
2. As such we will be on a mission to purify her with the help of the Ascians to quell a rebel from their group/someone who choose to use Hydaelyn to take ultimate power for themselves.
3. This is the reason why we ally with them or someone from their group and "embrace the darkness".
4. The individual who has done this to Hydaelyn comes from the empire or even someone close to us.

Again, this is merely opinion and please do not attack. If you have a differing one, I would love to hear about it.