As a breton, I can't let that one pass. His name was Arthur (which was all the more important given the signification of that name both in Brittany and Britain).
As far as I can tell, Duchess Constance had him named Arthur to deliberately exploit this; the Plantagenets had been using the arthurian legends for propaganda purposes to justify their dominion over Wales and Brittany and now there was an Arthur, raised by a breton court and friendly with the french, with as much of a claim on the english throne as his uncle (at the time it was still largely Gavelkind, or at least that's how the english justified having the already adult John pass in front of the very young son of the duchess of Brittany). John I's first worst nightmare until the barons' revolt.
The chronicler Walter Map wrily commented on the fact that the bretons now had a real Arthur when the whole thing started to happen.
I also agree there should be a way to have Sicilian flavor, Frederick II was one of the more fascinating Holy Roman Emperors. His son was already mentioned, what with his triumphant march through Italy.