I do have an idea for an IJA Master. I had something in mind involving him, Berserker, and possibly a Master of the German Faction.

Basically, my IJA Master could have been in contact with Germany as a spy or some other sort of operative. Either a German faction member could have given him Fafnir's catalyst, or knowingly letting him steal it, with the intention that they will easily beat Fafnir since they have Sigurd in their ranks. IJA Master then gives Berserker to one of the founding families who don't have a sufficient catalyst to summon a Servant.