I will buy this book eventually, probably as a trade paperback when it comes out that way. I've read all the other stories (except Zoo Day) in it already (AAAA Wizardry was released with the tabletop RPG, I think. I ahem found it online somewhere)

Maggie's part of Zoo Day is essentially a pilot for the Maggie/Mouse young adult series that Jim is planning to write with his... sister, I think? Seems very Harry Potterish, from the description.

Mouse's part could have been foreshadowing for those books, or for Harry's storyline. More likely the Maggie/Mouse stuff, if it happens.

Minor spoilers below, if anyone wants to avoid them.

Mouse's understanding/explanation of Maggie's villains seems unsatisfactory. How can they be some kind of coming-of-age monsters that are there to help kids grow if they can effect adults? Those baglers feeding on that elderly couple are there forever, it seems. Maybe adults have the ability to rid themselves of the creatures naturally, somehow? Maggie seemed totally unconcerned about them - weird that she (and all children?) seem to think even adults like Harry would not believe in them.