Actually I could see it in reverse.
Arturia (Saber) as Miki/Kaitlyn's daughter would be obvious for one glaring reason: who' the one that in games, anime, and manga....has a plushie lion she still sleeps with as an adult?

Caster would be great though as Miki/Juri's daughter. Especially once you see how she acts in Fate/Hollow Ataraxia.

And if we're having a Rider show up...we need Sakura too. Twins perhaps?

Gah, Gryphon has so many duelists (which is a good thing) and other melee experts running around, it'd be almost too easy to arrange a setup so that all the characters from Fate/Stay Night, Fate/Hollow Ataraxia, and Fate/Zero show up. Now if only Archerko (Sword Dancer's non-H doujin) would show up too....