Favorite dcmk head canons, go!
Shinichi has an American accent, because Hawaii, and he hates it. it's not an English accent (AKA the Sherlock Accent) so when he finally meets Saguru as himself, there is a lot of subtle poking for Saguru to speak English so that Shinichi can learn the 'proper' way.
adding on to this, Shinichi is totally an anglophile (fond of, or greatly admiring, Great Britain).
Kaito is really good at Latin dances; since they require strong core strength, teach body isolation/movement (move parts of his body independently), and increase stamina/strength/flexibility, they're very useful for KID-related activities. putting all that aside, he first learnt it for the flair.
Haibara never takes the antidote. a widely accepted headcanon but important nonetheless!!
Heiji and Shinichi's strange almost-romantic bromance peeves Kazuha, especially after she and Heiji start dating. however, she meets Shinichi and Understands. doesn't mean she likes it, but she gets it.
adding on to this, Heiji tries to establish a Thing where 'it's not cheating if it's Kudou'. Kazuha fights back by proposing a secondary Thing where 'it's not cheating if it's Ran'. Heiji immediately gives up, but Kazuha makes a point of being even friendlier with Ran when Heiji is around for the next few months afterwards.
it's not all the characters, but these are ones that've been stewing in this brain of mine for a while. thanks for the ask, @kosakka!