He might be the EC of Reikland... or maybe not. Wizards are hated and feared. Things could go very bad.
Assuming he doesn't die or get maimed with Marks in training in a way that would make him incapable of ruling, which is a very real possibility.
Ranald is far more risky, @Boney confirmed it. The nobles of Reikland will at least be able to bear a wizard heir, a secret Ranaldite heir on the other hand would likely face rebellion and death if found out. By contrast Nullify is something Mathilde would think of as crippling the kid since she thinks magic is a blessing.
Fair enough, I'll edit out the Ranald vote.