I don't think Subaru can be KID, Bourbon or Shinichi's father:
1. If Subaru is Kid, the attraction of Detective Conan will decrease, and people will focus on Magic Kaito more so that they can find out the reason. And in Detective Conan, Kid appears just to be Conan's enemy, not to take revenge for his father. As you can see, from episode 1 of Detective Conan till now, there's no Kid's background, so the fact that his father was killed by MAFIA is not true.
2. If Subaru is Bourbon, it's not logical. In episode 60, Haibara smelled MAFIA's person, it's because Akai was in MAFIA 2 years ago, and at that time, Haibara was also in MAFIA as Sherry. So if Haibara smelt MAFIA's person in that episode, there's nothing to be confused. Also, Haibara never heard of Bourbon, so when she smelled MAFIA's person in that crowd, there must be someone she knew when she was in MAFIA
3. If Subaru is Shinichi's father, when Subaru saw the fake Akai, he won't be surprised. Because Shinichi's father never saw Akai before, so even when they met, they won't have any amazement.
4. Subaru is as smart as Conan, he's the man of action. At first, Conan didn't recognize Subaru is Akai, until he said:"Black is my favorite color, because it can hide the true person that humans don't want to expose, and I also hate it because of that". Black here implies MAFIA, because he mentioned it twice. And also, he give Conan a second clue that he loves Sherlock Holmes. He knows that Conan loves Holmes too, so he said that to get Conan's attention. In addition, when the case is close, the kids mention going to the Professor's house. Subaru also knows the Professor because Conan had mentioned him before (when he suggested using the Professor's car to take Rena away, episode 58). Consequently, he asked to come to Professor's house. And that's the last clue to make Conan recognize Subaru is Akai. So when Conan heard that there was someone likes Akai at the supermarket, he was very surprised, not because he thought Akai was dead, but because he knew Subaru is Akai and he is living in his house now.
(I don't know if it's true or not, hope you can use it as a consultation
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