welll, never saw it anywhere soooo, most of the ppl think that shirou magic comes from mutations(due to excalibur or not)or maybe from his ancestors.
but what if his powers came from gaia? think about it, we know that the throne (of hereos),wich is part of gaia,transcend time,but could it trancend reality,"timelines"? could there be one throne for all of the timelines?
then we know that archer became a counter servant in exchange of power to save people during in a natural catastrophe. what if this power was shirou's tracing, confered to all shirous's from all timelines by gaia? meaning that archer didnt had his power during the war.
we're pretty sure archer shirou comes from one of the worst timeline where he's the only survivors of the grail war. meaning he performed badly, maybe due to a lack of power?(i know,all the dead end and all but...).
shirou noble phantasm basically have no limits with infinite mana, which gaia procures to all of her counter guardians. if gaia really gave him his powers, then she chosed weel (needed a tool of mass murder after all).
then we know that shirou can only replicate earthly ressources, if his noble phantasm was indeed bestowed by gaia, it would really make sense.
and yhe source of shirou magic is a reality marble,nearing the sorcery realm.i mean, i know he put his life on the line while training, and will plus emotions are pretty imprtant factors in the nasuverses magic system, but obtening a sorcery this way is not far form saitama's training level of absurdity. and gaia is the world, a marble phantasm create a world sooo....
then most of the other noble phantasm have really fleshed out background and dont pop out of nowher like ubw, i mean (he's basically born with this capacity,ubw being where he keeps all his "diagrams" its always existing inside him, and just refined it by training).
come, and destroy my theory ^^ (also im french so sorry for my english).