Cue a scene of Saber and Shirou running to each other in Avalon, only for Shirou to stumble and fall as she disappears when he expected to grab her. As Saber appears in front of Louise, she stares in shock while Shirou stares at the place Saber just disappeared from. And across space and time, all living beings hear two voices scream, "DAMMIT!"
Meanwhile, Louise realizes what she's just done and smacks her head against a wall repeatedly, groaning at the realization that she just totally fucked up her Servant's long awaited happiness. Thus begins the quest to either return Saber to Avalon without making it obvious that's what she's doing (I imagine Saber would still take her duty seriously enough to act as best she could) or bring Shirou back.
...You know, actually this could really, really be a fun thing to do. Saber as a Gandalfr, her interactions with Derflinger, everyone's reactions to meeting the woman Shirou loves, Guiche's reaction to meeting the person who showed Shirou his insane training method, Shirou's increasingly wacky attempts to get out of Avalon after spending years trying to reach it, and Saber's reaction to meeting Shirou's child.
God damn, this could actually be a really awesome idea. I might recommend it to GB.