Also, his spring fruit had sooo much untapped potential that it'd make a lot of sense for Oda to bring Bellamy back with newly developed gear-like powers.
Gear-like…. Oh lord, I hope not. >.>
But yeah, I voted for Bellamy. At first, I thought Krieg, but then I read all of the posts so far and Bellamy seems more likely to me now. He could tie in a Doflamingo appearance, and I'd say Luffy owning him in one punch after talking all that sh*t in front of all those people would be pretty devastating to him and his huge ego. Not to mention his defeat made Doflamingo lose any last shred of respect he might have had for him. So yeah, Luffy ruined his life. Meanwhile, with Krieg, it was really Mihawk who ruined his crew and his life, not Luffy so much.
Of course, I'm talking as if Luffy is the only one in the crew who could be the culprit of ruining Dupal's life, which isn't the case, so it could easily end up being no one we know or could have thought of.