With how this series has been going with people like Yukari and Eirin and Patchy as babies, I was thinking that it'd be a mommy Cirno with a baby Letty, but now...
Grandma Letty having to take care of teenage daughter Cirno and baby grand daughter Daiyousei seems like a possibility. Cirno will be getting bad grades in school because she plays with Daiyousei instead of studying, of course.