I mean I could imagine it being the last boss fight of 4.0. The climatic finale being against the Warrior of Darkness as his group of friends.
Though as much as they are being portrayed as an antagonist, I doubt we'll keep on fighting them. I think at some point we're going to reconcile our differences and figure out that they're being played by the Ascians. I can't recall a FF game where you've fought against the warriors of darkness.