Honestly I think I talk a lot about my favorite ones lol. Like, Conan's Kaito KID radar? Technically it's canon that Conan can sense when Kid is around, but everything else about it is headcanon territory, like how it works and stuff. There's also Akako's magic not working on Kaito because he's gay or trans (either he's just not attracted to girls OR Kaito's mtf and hasn't realized it yet but the magic immediately clocked Kaito as not a man hence getting exempted from it. Can't also help that Kaito really enjoys dressing up as cute girls. Just give it a few years or something).

Also that Kogoro is a competent detective but Conan solving the cases for him has made him somewhat lazy. I absolutely miss the days when Kogoro was suspicious of Conan and think it's a shame we don't get that anymore, so in my head I kind of just think that Kogoro is competent but just slower at piecing things together, so we don't actually get to see him shine much (and also why he gets very competent in cases that are personal to him, because in those cases he has a personal sense of urgency). He probably sees the whole Sleeping Kogoro thing as convenient for him and that's why he hasn't been as suspicious of Conan as before and that convenience makes him not really care into looking into it. Like "it's helping me, so why even bother".

Someone has definitely suggested to him that it's probably a medical condition AT LEAST ONCE, and he just laughs it off. He gets himself checked. They don't think he has a medical condition. And he's back to ignoring the issue again since the Sleeping Kogoro brings food to the table.