I was thinking of something like that. But Iago is actually the hero.
It's a switched morality game. A mysterious songstress comes and begins corrupting the middle child of the Nohrian royal family, tricking her into assassinating the queen of Hoshido. Iago, the loyal tactician of the King, would have to find a way to take down the enemy Hoshidans, and save the royal family from the corruption, defeating Azura once and for all. On the way, he recruits a misunderstood Berserker framed as a killer, a young boy with the ability to morph into a faceless, a Dark Mage with illusion powers, and a brave Master Ninja.
I think that Iago had a lot of potential that they wasted. I mean, we don't have a good reason why he hates the avatar. He even tries to get them killed when they're technically on the same side.(and with their siblings and retainers, which means he's risking the lives of the best warriors in Nohr over a dumb grudge)