They haven't announced a price or date, so no point in complaining just yet. It could be a preorder bonus for all we know. And honestly, Ezio doesn't belong in the Final Fantasy universe anymore than Kratos does, so I don't mind this being optional DLC.
Now if they announced that Cloud, Sephiroth or some other core FF character was only available as paid DLC then I'll probably complain about it.
I do understand where people are coming from, as it often feels like some DLC should have just been included with the game in the first place, but we also don't know whether or not this content would have ever been created if not to sell as DLC later on.
So yeah, I'm split on the idea. I don't want to get ripped off, but I also like getting more out of my games by installing (some) addons.