I expect the Warriors of Darkness to be the heroes Kidibus mentioned... They've been corrupted, Y'shtola comments on how the aether is strange around them, and their first act? Slaying Ravana... Why? Well, if they're the heroes Kidibus mentioned, they'd be slaying him for his power...
That would put Kidibus in an interesting position, too... Presumably his master is Elidibus, yet Elidibus is also using the very power Kidibus warns us about... Plotplotplot, Kidibus realizes he is being used (because we can't justify fighting such an adorably clumsy kid!) and that using Primals isn't inherently bad, you just have to be smart enough about it to avoid the negatives, 4.0 roles around and he is in charge of us summoning Primals as a Free Company for whatever glorious Campaign-esque content can justify it...
On Ala Mhigo; Ilberd is a pretender to the throne, using that mask to claim he is Theodred to incite rebellion. The Masks have their first battle, actually win, Y&P find out it is Ilberd and come out of hiding and inform us of what's going on. Garlemald wants to stop this rebellion, we want to stop Ilberd, we go to Ala Mhigo under an uneasy peace and Garlemald gets fleshed out a bit more, rather than the current "They're literally Nazis" narrative they have... It would be nice to have some kind of ultimatum with them; If we don't help, they'll be forced to come down hard on Ala Mhigo, meaning the Eorzean Alliance has to worry about that... Alternatively, if Ilberd is successful, do we really want him leading a liberated Ala Mhigo? Given how royally Ul'dah screwed him, I expect he'd start the Autumn War II... It is in Eorzeas best interest to go to Ala Mhigo with Nero as our Garlean envoy... I'd expect something similar to XIs Treasure expansion, where we go there to help out, but we're still reporting back to our side... Maybe have Ilberd find another Allagan Weapon, giving us a nice Diamond Weapon battle at the end of 4.0, since every final boss has to be a FFVII reference... Garlemald can even blast it with their new Eikon Cannon, all the way from their homeland, letting 4.x lead into a new conflict with Garlemald ready for 5.0...