Shinichi is Yusaku
So imagine that Yusaku is the one who's forced to take the apotoxin 4869 (say, after a Kid heist) and he pretends to be his own son (really convenient for hiding from his editors and thus, Kudo Shinichi is born). By the time the manga roles around he's a teenager and never found a cure. He knows who Kid was and that Kaito took his place so he's more active in the heists (whether up front or in the background). I haven't seen this yet but just imagining it is fun. I imagine that Yukiko and Yusaku's marriage was more for convenience to satisfy their parents and follow their passions; they're friends rather than lovers. That way Shinichi and Kaito can be a pairing (Yusaku's gotten used to being a teenager again [all the hormones...] and Yukiko has always wanted to see Yusaku and Kid together).