After a few days of nothing happening, something happened to make me think, “Hmm, you know, Julian Ainsworth from Prisma Illya could be a Master in this.” He’s got a crazy wish to save the world that could easily be displaced into this, with him recognizing the world’s declining state and wanting to be the hero who uses the Holy Grail to do it. He’s got an interesting enough skill set with Flash Air (basically Kawarimi from Naruto except switching anything with anything), even without letting him have any of the weird Grail related powers he has. Plus the whole “dolls animated by the spirits of the dead” could totally be used. The Ainsworth family is old enough to be able to justify having an artifact to summon just about any Servant IchaSennin wanted to use. And last and most importantly, the two Dolls he has already, Angelica and Beatrice…
I mean, Angelica is the closest thing we have to a canonical female Gilgamesh, so she’s always been great. But...this is official art, meaning Word of God, Beatrice Flowerchild do got the booty. And damn but she do.
Hell, could use Prisma elements with it, too. The Ainsworths picked up Miyu during the fire to use as a Grail to save the world, Julian is still working at that but also has her be a Master to try and stack the deck in his favor, but she escapes and winds up with Shirou, eventually Julian tries something against Shirou, winds up losing Angelica and Beatrice, too. So basically the same thing that will happen to Kayneth could happen here, with Julian losing his servants and everything to Shirou.