The Nasuverse TTRPG is a tabletop game based around the works of Kinoku Nasu. One compiled document, by Twogi Short Description I had time at work, so I wrote, and then ran, then wrote while running, and effectively made a full TTRPG based on the works of Nasu Kinoku at the time. This was before Fate/GO took off, so visual novels and Melty Blood was all you really got in that universe. So when I got my first job in Korea, and I had tons of time to read through visual novels and play the doujin games, I got -really- into the Nasuverse and modern fantasy overall. The kind of uber-nerdy fan stuff you get up to when you're in college and you don't want to do CS homework.
Basically a total conversion mod, with me taking DnD 3.5 classes and adjusting them to be like the magatama from that game. My first one was cribbing SMT: Nocturne into d20 and running a full campaign on it. So I've always loved TTRPG design and philosophy, and have dabbled for ages in homebrews.