The Borrower Arrietty/WMG

Sho/Shaun grows up to be Howl.
Spoilers for The Borrower Arrietty, with facts from both the book and the movie versions of Howl's Moving Castle.

At the end of the movie, the ending is ambiguous (Japanese) or he lives, (American). I posit that the answer is both, that is, he died in Japan, but he lived on in another dimension, the dimension from HMC.

Sho's surgery didn't work, and his heart was going to fail early, and he was going to die young. Hearing of this, his estranged uncle (maybe "Jessica"'s estranged husband?) from a different dimension heard about it, and decided to bring his dying nephew to his dimension, and the house and the flowers, so he'd have a chance to see it before he died. While there, though, Sho found about the falling stars, and made a deal with Calcifer when he fell. Calcifer kept his heart (and him) alive, but Sho's personality changed due to a mix of his heart being gone, and actually having a chance to live.

However, because he didn't know if actual magic existed in his world (oh, yes, there were the Borrowers, but did that mean magic?), so he "died" in Japan, and decided to live the rest of his life in the dimension where he could learn magic, and where his heart was "safe".

Years later, "Howl" makes a name for himself, and meets Sophie, and the events of the movie happen. Now, in the book, Sophie was able to speak life into things, and will things to happen because of magic. In the movie, Sophie does the same thing, unconsciously, and that along with years of being magically healed by Calcifer, his heart was fine, and he lives the rest of his life with Sophie.


 * Note, in the movie/book, the uncle's place is somewhere where Howl goes during his magic lessons to get away from Sullivan, also, Howl was already aware of magic, and an apprentice to Sullivan when he went to make the deal. This WMG could still work if he was apprenticed to Sullivan as he was dying, as a dying wish, but he would already have to be learning magic before he traded his heart away.

Sho grows up to be the uncle (and childhood guardian) of…Shinji Ikari.
Sadly, it turns out that someone with experience being virtually abandoned by workaholic (albeit equally emotionally wounded) parents; the traumatic fear of trying to connect with other people out of lonely hope, but only managing to expose both to being hurt in the process; the firsthand knowledge of fantastic, eldritch worlds and life existing secretly alongside our own, rich with joy and togetherness thought unimaginable, if not for the meddling of individual humans, and a resigned familiarity to one's own mortality (often finding himself bedbound on doctors' orders), while undoubtedly sympathetic, probably isn't the best choice to foster raise an equally-traumatized young boy. For anyone.

It also starts to indicate family propensities towards gravitating towards brash ladies, dolls, and cat-loving daughters of (apparently) crazy researchers.

The Haru shown here is also the Haru from The Cat Returns.
She became increasingly neurotic after leaving the Baron and became obsessed with small talking creatures.
 * And maybe she thinks the Borrowers are disguised agents from the cats to bring her back to marry the new cat prince.