Imagine Hiro getting "incinerated" by one of his own inventions. There are scorch marks on the ground, and Tadashi is horrified. Is Hiro dead? Hiro has been shrunk down to borrower size and is living in the walls and avoiding Aunt Cass' broom. Tadashi doesn't find out he's alive until he's reinspecting Hiro's invention and gets shrunk himself.
HEEEEE TEENY HIRO. Well, teenier than usual Hiro
Oh gosh, and imagine them going on adventures, riding Mochi around like a really fat noble steed. And Baymax is the only one who can find them, but that’s only after his scanners have been recalibrated for really small things. And in the end Aunt Cass nearly eats them by accident, because both her boys have gone missing now and she’s stress eating, and they fall into a donut, and only Baymax or Mochi manages to save them. And then Aunt Cass spots them and faints, and Baymax has to help her, and then when she wakes up she gives the boys the talking-to of their lives, and then they all realize how ridiculous the situation is so they just start laughing and laughing.
Aunt Cass and Baymax help fix Hiro’s invention so that they can re-grow the boys, following along with Tadashi’s and Hiro’s directions (maybe they ride on her shoulder so they can speak loud enough to be heard near her ears). Thankfully Aunt Cass has picked up a few things from having two genius boys for sons, so it’s fixed quickly and everything goes back to normal.
And they all agree it’s Fred’s fault because he’s the one that asked for Hiro to make him a shrink ray in the first place.