Here's an idea that doesn't seem to have been proposed before: Genji Kojima. (Genta's father.)
Sure, take a second to laugh if you'd like, but he fits.
He owns a liquor store/bar. (I seem to remember seeing both, so one is likely a case of bad subs.)
If his age has been affected like Vermouth's has, it could explain why he speaks in an old dialect.
I don't know when his name was first mentioned - though IIRC it was somewhat earlier on - yet he didn't appear until episode 551/chapter 658. (Wiki link)
I'm getting ready to rewatch the case he appears in now, just to see if I can pick up on anything else. Naturally in all likelyhood it isn't him, but I wasn't going to let it be overlooked.
Edit: A few things did jump out at me after watching it again, some that would seem to support the theory and others that would seem to oppose it.
For:
He purposefully changes the way he speaks, reveiling his normal speech to be a facade when he gets in a fight with the culprit.
He wears glasses and squints his eyes during the case, despite not usually wearing glasses. (Keep in mind that both Shinichi and Akai turned to glasses when they needed to hide their identities, so this could be a clue by Gosho or simply something he likes to use to complete a dsguise.)
Maybes:
He seemed to be proficient in some sort of martial art, as he was able to knock the culprit to the floor in an instant and pin him down. However, it was a surprise attack, so that may be meaningless.
Genta says his dad is combative, honest, and hates dishonorable things, and the idea that his dad is a good person is really pushed throughout the whole episode, almost so much to make me think the opposite.
Add to that the extreme aggressiveness and intimidation of the scene where he picks a fight with the culprit, and the above seems even more suspicious.
His favorite song is apparently Kuroda Bushi. I have no idea what this would translate to, and it's likely meaningless, but the fact that his favorite song was included when the only clue we have as to the Boss' identity is a song struck me as odd. (And at the same time, for all I know Nanatsu no ko and Kuroda Bushi could share the same notes, the same way Twinkle Twinkle Little Star is also the Alphabet Song and Bah Bah Black Sheep.)
He wasn't interested in winning the sizable cash prize, but instead in the boar scroll. The scroll was worth more than the cash prize, but the reasons he gave for wanting the scroll were that he simply liked boars, and that the artist's name sounded like "thank you". It's kind of hard to believe that that alone could make the scroll seem more valuable to him than the cash prize, assuming he didn't know the value of the scroll.
There was also a bit of a story/legend behind the scroll, which was of a set of three. Supposedly, having all three scrolls would gaurantee good fortune and success. While even I think this is unlikely to be a story that'd catch the Org.'s eye, they also seemed interested in the tale of the immortality arrow.
Against:
All of the talk of Genta's dad being honest and honorable could just as easily be true as not, making him a poor boss candidate.
While only briefly, Haibara (along with the rest of the Detective Boys, Conan and Hakase) does walk with him, even if she does stay a few paces back as per usual. Her Black Org. sense never kicks in, and he doesn't seem to recognize her. (And she isn't dead, so...)
While he was masking his typical way of speaking and wearing glasses that forced him to squint, he was still planning on going on T.V., which would seem like something that an overly cautious person wouldn't do.
Well, that's all I could gather at least. Anyone have anything else to add?