The reason I'm saying lantz should write original fiction isn't because it's bad (which it is), it's because then I can stop trying to treat it like it's in any way related to the Nasuverse. It's not about AU, or raping established canon, it's that he flat-out refuses to adhere to the established mechanics of that universe and all the worldbuilding within it. The unwritten rule of writing fiction of any decent caliber is that you obey to the rules of the universe you write within. Urobuchi didn't go "I'm doing it anyway because it sounds cool" when Nasu told him to not make a Xian a Caster for Zero because that defies the established mechanics of the Grail (read: The Grail cannot summon spirits from "the east", meaning China, Japan and anything else east of India. Before you say "but what about the Moon Cell?", Moon Cell works 100% differently and Medea got Kojiro by forcing the Grail to give her it's best shot; a nameless farmer who resembled Kojiro), he went "Oh shit nigga, lemme find someone else" and got Gilles.
Even if lantz tries to handwave that "Satoshi predates the now-translated material", it still puts him in 100% defiance of the things established in the Nasuverse (MA, an organization that's been around since the Age of Gods, being a world-spanning group of Seiba fanboys that will WILLINGLY hand over control to a Magus from a backwater country because Seiba-sama said so? What the hell were you smoking, and where can I buy it?).
TL:DR: I refuse to believe that lantz actually has interest in writing a Nasu fic, and am hoping from the bottom of my heart that he is trying to get legs on a original fictional universe instead of ass-raping a canon that I love dearly.