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I didn’t see anyone else do this, but I have many thoughts about an Arcane Fate AU.
The concept is that Hextech and the story of the two sisters took place roughly 100 years ago, and during that time, Zaun gained independence. Because of this, characters like Jinx and Vi are now historical/folk figures in Piltover and Zaun.
The Holy Grail Ritual is a fancy competition between Piltover families to show themselves off. They usually select representatives with sufficient magical abilities to compete, and they use Servants as familiars in battles. However, in this particular Holy Grail Ritual, something went wrong, and some Servants ended up being summoned by unintended individuals (people from Zaun).
The Master-Servant pairings and classes would look something like this
Piltover:
Viktor - Jayce - ????? Caster
Mel - Lest/Leblanc - Assassin/Pretender (what i mean is Leblanc is pretending to be Lest and that her class is assassin)
Caitlyn - Vi - Saber or Shielder i am not sure about this
Zaun:
Isha - Jinx - Archer
Ekko - Heimerdinger - Rider
Singed - Warwick - Berserker
and special guest:
Ambessa - Arcane Herald - claiming to be saviour those who are more familiar with the fate series could probably guess his true class