The Doujins will end with Chiyo falling pregnant:As the Anthropomorphic Personification of biological reproduction and the trope namer for Mother of a Thousand Young, it would be stranger for her not to conceive with Yuu.
Yuu has inherited his mother's health issues:Several chapters have Yuu suffering fainting spells. The reveal of his mother's frail health suggests it may be a hereditary issue (all the more plausible considering his parents' inbreeding).
Haru is a Hound of Tindalos:As evidenced by her self-proclaimed canine nature and Chiyo threatening to find her "no matter where in spacetime you run" if she ever hurts Yuu.
Yuu's name has been mispronounced all this time and his surname is Narukami:The reason why he is so inept and quickly adapt to his personas wasn't just because of Izanami's experiment, it's also his time that he spent with Chiyo and his overall nature to attracts and feel the weirdness/supernatural forces around him that quickly make him a formidable and a bit of weird magnet. The "parents being abroad" are just a one big excuse and Dojima knows this, not to mention that Shub Niggurath exist in Persona 2.
- Chapter 41 confirms the family name to be "Hasunuma", so that's out the window.