During the arc about Luffy’s childhood, Dragon decided to restock at a dojo. Why would he restock at a dojo, unless they were working together or because Koshiro is a revolutionary? Why else would Koshiro help someone as important as Dragon, who is hunted by the whole world and is considered the most dangerous man in the world? Also, we know very little about Koshiro. The only thing we know for sure is that he is a very good swordsman (though we don’t know how strong he really is), he knew how to cut steel (during Zoro’s fight with Mr.1, Zoro remembers Koshiro’s word of what it really takes to cut steel). Also, he did have a dojo and he also trained Zoro and Kuina, who were strong, so he, himself, must have been strong as well. So, he could be a powerful ally to the Revolutionary and to Luffy, if he ever joins forces with the Revolutionary. So, it would be Luffy with Dragon and Zoro with Koshiro and they would be quite powerful. And we also know how Oda-sensei likes to connect things. Also, Koshiro might not be a Revolutionary, he could just be Dragon’s good friend, but I think that would be, kind of a waste. And one more thing, if they don’t have a connection, why bother and draw that panel in the first place. Knowing Oda-sensei, nothing he draws is coincidence and everything has a meaning and a connection to something. All rights go to the maker of this theory.