When you look at the general picture, the first thing Aoyama-sensei wants you to believe is that Okiya is Bourbon, because of the big hint-dropping around his first appearance that suggested this idea. Okiya and Bourbon are both very smart, Okiya has been seen drinking Bourbon, the B.O. aura that Haibara feels around him, etc. The theory is still a little unstable since there isn't enough evidence pointing to it. For the sake of my deduction, I'll drop this idea and say Okiya and Bourbon are two different persons.
I think it's safe to assume that half the DC-fanworld is more or less convinced that Okiya Subaru is in fact Akai Shuuichi. Theories with many arguments have been solidly established, making it hard to believe that Okiya could not be Akai. (But that's only assuming Aoyama-sensei isn't planning on coming up with a new unexpected twist where Okiya is actually Shinichi's long lost twin brother in disguise). A coincidence is also very hard to believe. Since Okiya being Akai is rather likely, we'll keep this idea.
Bourbon could be pretty much be anyone at this point. It could be someone we know. It could be someone we don't. It could be someone who's been present the enitre time, it could be someone we haven't seen in decades. It could be something absurd like Eri's cat Goro. Or it could be a very smart guess.
Leaving Bourbon aside for now, let's get back to Okiya and Akai. We're assuming Okiya and Akai are the same person. But they really are, that begs the question:
Who is the Akai with the burnmark?
Okiya Akai and the fake Akai have been seen together in the Red Shirt case (files 700-704). It can't be Vermouth, since she was with Gin outside at the time. And for all we know, there's no character that would gain any profit by disguising as Akai and walking around town as if nothing were. If I use statements that have already been made on the forum by other users, such as the real Akai being left-handed while this one is right-handed, we can pretty much acertain that the Akai with the burnmark is not the real Akai as we know him to be. So who is the person disguised as Akai?
...Unless it is not a disguise at all?
There is no evidence pointing to the fact that it really is a disguise, it's just a natural assumption one would make because he looks like the Akai we know. With this in mind, the fake Akai could be anyone, from a new completely new character that sudenly shows up to a character that we heard of. If we cross out the idea of "new character", that leaves us with "someone we heard of". Which also means someone we don't know what he looks like.
With this, there are only two possibilities left that come to my mind.
1- The fake Akai is the Boss of the B.O. Maybe. Why not, it's possible, but not very probable.
Therefore my best guess is:
2- The fake Akai is Bourbon.
I don't really have any evidence to back up my claim, aside from their mutual reactions at the end of file 702
In the end this is only one big guess with no solid evidence, but hopefully a good guess at that. What do you think?