I've seen quite a few people with that idea of Higuma, Ivotas. Boiga's post about Bellamy really has me convinced that it might be him.
Gin was mentioned as a candidate. I really don't think its him simply because how did the SH wreck his life? They beat Krieg and its not like they went to jail or lost their rep. He just ran away like Moria and crew did albeit in front of some witnesses. Gin never seemed the type for revenge. Sanji was way too kind to him and honestly I still think he feels responsible for Kreig.
I considered Kreig as well. Armor fits the criteria, etc etc. However, I just don't think it would make sense for him to come back this way. Again you kinda gotta think about the most crucial phrase here: Ruined his life.
Kreig's life was kinda headed downward when Mihawk owned him and his crew but Luffy's defeat didn't get him arrested nor did it lower his bounty or what not. Only his crew, the SH, and a bunch of Sea cooks saw his defeat. Not exactly a life changing event.
Morgan, I love the idea of him being Dupal but as said I can't see him taking away Helmeppo's dream. However, this would be an awesome twist. Again though, consider that every main villian in an arc since the GL began has had a DF. (Am i forgetting someone?) Morgan doesn't and unless he found one there is just a limit to his potential. He knows Luffy has one and i just can't see him improving to a huge extent. Plus he is on the run from the WG when would he have time to train?
Now as I said Boiga's post on Bellamy really sold me on the idea. For a long time I had this kinda idea that the main villian of Fishman Island would be Doflamingo (I base this on nothing but a feeling) and i believe that the "Paulie/Wiper" character would be Jimbei. Again baseless speculation. As far as how Bellamy beat them there? Well actually thats not too hard to guess.
Consider this, Doflamingo is a Shichibukai right? He might have access to better and faster routes than a normal pirate. Remember that honestly Oda doesn't kill off Major villians he just shatters their dreams. I think Bellamy is alive for this reason. (Yes I know Mr 11, blah blah) Bellamy has all the right reasons for revenge here. He has a potential to grow and his face might be disfigured as mentioned. Its been so long since Ive seen Jaya i can't remember his skin tone but wasn't it a little tan? Again speculation.
Honestly, I hope two things: 1) That this isn't the main villian and is just like Franky was at begining of water 7 , a secondary villian and 2) I hope this is someone we know from before. I love new characters and all but Oda writes such awesome characters that I'd love to revisit an old villian.