actually I feel like if Caster!Cu would have a noble phantasm, it would be that fortress noble phantasm that's mentioned in Fate/complete material III!! As a Caster he'd be able to readily created bounded fields that he could potentially declare as part of Ireland (this is iffy and speculative) since apparently it can only be used there, or maybe he could have personal mana sufficient enough to conjure it as a Reality Marble :o
that’s an interesting thought but something on the level of “declaring a bounded field a part of his homeland in order to allow the use of a noble phantasm” sounds more like something a conqueror type heroic spirit would use rather than a defender/warrior type like cu.
also, the whole power boost thing is based on the people who know about the heroic spirit that live in the land rather than the land itself. heroic spirits are highly conceptual existences that rely on being known in order to exist, to the point where heroic spirits that didn’t even actually exist can be formed purely from people believing they did (sasaki). it’s not something in the soil of ireland or w/e that gives cu the power to use that stuff, but rather the sheer amount of people living in ireland that know about him.
a reality marble sounds interesting too but its an odd choice for a character who is a warrior before he is a magus, and only ever specialised in runes at that. I doubt cu would ever consider himself a magus at all, he’s just a warrior who happens to be pretty good at magic. iskandar is able to have a reality marble is because it is sustained by all the warriors inside it at once, and it’ll fall apart once there aren’t enough left inside. cu has nothing of the sort though, I don’t think he’d be able to create let alone sustain a reality marble of any kind.
