Yes, they do it on storyline purposed, but just because Alexander has always been a castle doesn't mean they can't change his look by, say, changing his shape, or making the avatar that possesses Alexander the one we summon. Alexander is just a puppet powered by energy from the Astral Plane which the Empire control. When he got "mad", he got a kind of free-will. Probably always had it, but the Empire had built him so that they could control him. Eventually they lost control, and and Aphmau broke the controller.
Basically, Alexander doesn't have to be a castle, as that's just the shape the Empire gave him. Odin has always been fighting Alexander, so he could very well be a man on a horse as well, or a Qiqirn for all SE cares. They don't have to stick 100% to the previous games. Otherwise Odin's horse would have 6 legs like in the previous games (nevermind him having 8 in reality). Need I remind you that he also had wings in FFIX? Hell, Atomos don't even have the same shape he had in some of the previous games. For all we know, the previous games Alexander were just a puppet possessed by an Avatar too. He's always been a giant machine or a town, so Alexander is really whatever possesses the town or machine, whatever controls it, like electricity.
edit: no, I'm not saying Alexander in castle form will be the new avatar. I'm saying that theoretically SE could make what's controlling Alexander to be the new avatar. Personally I believe Alexander's corpse will be the key to the new Avatar, as SE really built up for a storyline involving it.