@Smudger:
Well I guess it might not be 'THE' main story, but we've had a heck of a lot of his past covered, which is predominantly made up of key moments in his life. While I have no doubt their is more to him, I cant help but wonder what, or where it will even fit in among his timeline.
Do we even know when he got his scar? or at what point in his life he came across/discovered his ambition?
He got the scar sometime within the last ten years, after becoming a warlord and leaving FI.
If it was just a meaningless timeskip battle damage scar, like we see on Coby, Mr. 1, Smoker, or even Zoro, Oda could have added it at any point along the way during the flashback as decoration. But since he didn't… its clear he took it off and kept it off for a reason.
As for personal ambition, it might be in plain sight, People keep going towards "harmony between fishmen and humans" but I don't think that's his PERSONAL ambition... that's purely something done by setting example that will take generations and is more Otohime and Shirahoshi's thing... and he doesn't realize he's a character in a story, so he doesn't know traveling with Luffy would do that) He was conflicted between Tiger and Otohime's paths anyway. A lot of folks also point towards "Honoring Ace", and sure, thats a reason to start watching Luffy, but its certainly no reason to travel with him as a friend.
His personal thing might be a nuanced thing that's in plain sight that will be obvious once we know what it is or get the last piece or something we haven't really seen yet at all, since really we've only seen him at absolute crisis points and in hasty recap. (As much ground as the FI flashback covered, it sipped through several years in a hurry).
While he certainly respected Whitebeard enough in the flashback, he didn't have the fierce loyalty he possesses later. And of course he met Ace somewhere during that time. So... it may all be tied up into that. (And that he got his scar AWAY from FI seems to hint at it being something else as well.)
Of course, there is the fact he needs to just let go of his responsibilities, since he carries the weight of... everything.
So, honestly, there's no telling what it might be. While we knew her navigator skills and hatred of pirates, we didn't learn Nami's dream of making maps till much later, Usopp announced his dream on the spot well after he joined. Franky travelling with a dream ship was only mentioned right before he joined, and Brook/Laboon only had the subtle clue of the timeframe of how long he'd been alone until that was dropped on us, and even Robin, who we knew had a thing about the true history, we didn't know WHY for quite a few years. (And finding friends wasn't really obvious on her until later either.) Really, Luffy, Zoro, Sanji, and Chopper are the only ones we've known explicitly up front what they wanted.
So while it might be fishman/human relation related due to the key figures in his life dying... I think its probably something else that we've only gotten the barest hint of at this point.
Side note: Why doesn't your quote show in my notifications?
Dunno.