Oh damn this is an interesring first choice. Guns ablazing, no obscure dream votes or any such nonsense.
I say give it to Shiroe freely. I figure if he pulls Avalon out and learns proper magecraft then we'll either not get a Saber Artoria bein used against us, or at least have a notpranastarved Artoria and a friendly Fujou Master to ally with.
If (as I hope) extracting Avalon means Rin doesn't get Archer EMIYA and we already sealed or weakened Herkzerker, then potentially we'll be in a war where only Lancer and Caster and maybe Assassin are the same.
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Oh yeah, given what the UBW booklet revealed about the einzberns and how the family literally just packs up shop and shuts down when they lose Ilya, how are you going to make this new info jive, Alf? A year of angry pursuit until they realise Ilya is done and then they abandon ship or something?
I have to be honest, a Grail War without the Einzbers would be rad.