If we're throwing out precise hypotheses (or theories, if you want to call them that)...

With the clips of the Griffin talking at the end of the trailer, there is still a shadow of a doubt (always will be until he takes off the mask) but it's fairly safe to say Ilberd is definitely the Griffin. The voices are similar enough, running on my memory. (If they hadn't recast everyone but Aymeric and Haurchefant I'd be far more certain...)

1. We're going to be called for a meeting at the Lotus Stand. We'll be informed the Garleans are about to go on the offensive and try to acquire Omega from the Cartenau Flats with too massive a force to counter. Nero will interrupt and suggest the GCs intervene as well, but they will decline to do so for whatever reason (lack of manpower, neutral territory, take your pick).
2. We're sent to repel the invasive force. This is where we meet the Garlean Roegadyn officer with a gunshield and gunhammer. Nero will intervene with his souped-up red magitek armor, and we will manage to repel them before Cid saves us on the Excelsior.
3. After returning to safety, Nero will suggest he and Cid return Omega to operational status for a contingency plan. Just in case. They do so - this becomes important later.
4. The Masks are going to rile up trouble on Baelsar's Wall, causing the Garleans to go on the offensive against all of Eorzea and necessitating military intervention from the GCs. This is a setup by Ilberd, who will use the aether from the soldiers' deaths and the Eyes, along with the faith of the Ala Mhigan refugees, to transform himself into a primal (of Rhalgr?). (This... this I cannot forgive.)
[5.] Papalymo will use Tupsimati to try and seal away Super Saiyan Ilberd like Louisoix did Bahamut, but like Bahamut it won't be enough. Cid & co. will arrive to extract survivors on airships.
[6.]Restored Omega will have to blast Super Saiyan Ilberd with its main cannon to stop him. (Note: it looks like it's Yda hitting the button to fire the cannon, and in the same scene there's another person standing behind Cid. I can't make out who it is exactly, but from what I can see it looks like Dragonlancer's armor - which would mean it's Derplander, and by extension that we'll be there when they fire the cannon.) The blast will annihilate Baelsar's Wall, opening the door for us to go into Ala Mhigo.
7. 5.0. Then ???.

5 and 6 are interchangeable until I know more. Or perhaps we'll have to shoot Super Saiyan Ilberd with Omega because the best we can do with Tupsimati is bind him for a few moments? I dunno... I'm kinda tired, and probably wrong anyway. Maybe Baelsar's Wall happens early, and reactivating Omega will be suggested by Nero as a last, desperate move against Super Saiyan Ilberd? Considering his nature and the way the EN dialogue tends to be a bit vague, I'm not sure if Nero's being literal or poetic when snarking at the GC council how they always send us in and "pray the world is not engulfed in flame."

I'm just not sure, since a lot of characters who appear in the Baelsar's Wall scenes are also in scenes surrounding Omega and the White Dalamud. Both Yda and Derplander are present there, but also inside Omega with Cid and Nero when Yda slams her fist onto the button. Baelsar's Wall -> Super Ilberd -> White Dalamud -> Cartenau -> Omega -> Wave Cannon? And the blast is still what lets us get into Gyr Abania / Ala Mhigo proper...

Here's hoping this doesn't get as depressing as Danganronpa 3: Side Despair... but I got a bad feelin'...