I do have an idea who the real villain might be. I think it is another member of the Fishman Pirates (Jimbei's crew), not the New Fishman Pirates. Jimbei was captain of the Fishman Pirates, the former sunny pirates, who would have a strong connection to Fisher Tiger; As strong as Jimbei, Arlong, etc.
Unlike Hodi and the NFP who were just children, Jimbei's crew would directly have known Fisher Tiger. Notice that Jimbei's crew is not with him - he is alone at the Sea Forest. Fishman - Human relations at Fishman Island are very high…Decken and Hodi are troublemakers, but neither truly rivals Jimbei in power. "Boss" Jimbei is also called knight of the sea. He’s too honorable to let even a blood relative run around committing the same crimes as Arlong. He would NOT fight Luffy over doing the honorable thing, even if it was defeating his son in his battle.
Jimbei’s reason for warning Luffy not to fight Hodi is simple – Hodi is seemingly attempting to adopt the will of not only Arlong, but also is twisting the words and ideals of Fisher Tiger to rally support amongst the Fishmen. The name of Fisher Tiger would likely resonate with those on Jimbei’s crew who formerly served under Fisher Tiger. At the time, it is apparent Jimbei’s crew is not with him – but they’re not causing trouble either. Should Jimbei’s crew be on the fence…a human defeating Hodi, a man claiming to bear Fisher Tiger’s will would make him into a martyr for the cause of turning Fishmen against humans. By Luffy (or in this case Zoro) defeating Hodi, an even more powerful and charismatic leader will rise up amongst the Fishmen…from Jimbei’s (and Tiger’s) former crew; A former member of the Sunny Pirates and the Fishman Pirates (not the Arlong Pirates, New Fishman Pirates, or Decken’s crew), perhaps a former first mate (just an example)…perhaps the one responsible for either Tiger’s death (Tiger didn’t hate humans…maybe this true villain thought he went soft) or Shirahoshi’s Mother’s death.
A lot at the end here is just speculation, but I think it is not Jimbei, but certainly someone with deep connections to Fisher Tiger, perhaps someone who can sway Jimbei’s crew over to his side, perhaps Jimbei’s former first mate, perhaps Tiger’s former first mate, perhaps the one responsible for Tiger and / or Shirahoshi’s Mother’s death(s).
copy / pasted from another thread where it was proposed that Jimbei might be the real villain of this arc because Hodi is his son, so there are parts in this post that address that theory…they're not relevant to this discussion...but the rest is.
It was apparent from the start that neither Hodi nor Decken would be the big arc villain.
I like this because if it's true then what if Hyouzou is loyal to this other fishman pirate and is just with Hodi to make a few bucks.