I really appreciate how much Horikoshi seems to love his characters
I've been reading Horikoshi's first serialized work Omagadoki Zoo and ran into several familiar faces - the characters of Uwabami, Shishidou and most notably Gang Orca all got their start here before being reused in MHA. The character of Astro from his short-lived Barrage makes some cameos towards the end of the series, and while I haven't read it yet, apparently Shigaraki draws influence from Horokishi's very first one-shot called Tenko.
And that's on top of making references to the movie characters as well! I'm not sure how much of a hand he had in the movies or their characters/designs if any, but it's still really cool that he went out of his way to canonize them in his manga, something I honestly don't recall any shonen doing for it's spin-off movies like that. Maybe I'm reading in to things but it's really cool how he reuses characters to pay tribute to his previous series & to the people who worked on the anime and movies.