I'm going to guess the one's who will get a backstory will be:
Blackbeard-I mean, he's almost too important not to. And there are interesting things to find out about his childhood, given his special "lineage". This is almost a guarantee.
King-We need to find out more about Lunarians and he's the only one. I could see us getting a flashback about how he survived. And what happened to his parents.
Mihawk-Yeah, we'll probably get a little something for him. Maybe not since he was a child, but at some point in his past. I mean, his origin as a kid was probably "he liked swords". But, Oda's teasing some betrayal for him, which would be nice to see in the manga proper.
Urouge-This guy is still mostly a mystery. He's got a story. His story hasn't been told yet. I think there's a fair chance we get a backstory with him, explaining his "fall", as it were.
Crocodile-Yeah, we'll probably get something for him, just maybe not since childhood, again. We know basically nothing about his past, so Oda's probably keeping it a secret for some sort of reveal down the line. Maybe.
Kalifa-I feel like this is possible. Something's going on with her and Alpha. If their entire family has always worked for the World Government or if they're clones or something, there are things we still could to find out.
Smoker-If we get anything, it will probably be short. But, I do feel like he's going to return to the story, someday. And when he does, we might learn a little about why he joined up with the World Government in the first place.
Moria-Maybe? I could see Moria having a interesting childhood. And he's coming back into the story, once he gets rescued from Hachinosu.
Perona-Same as Moria, really.
Garp-I feel like he's got a interesting backstory. I just don't know if it's really going to be relevant to the story at hand, though. It feels like there's prime material for some sort of spin-off sidestory novel there, though.
Akainu-Again, there's probably a interesting story there. If we get anything, it'll probably be right before he dies, though.
Boa Hancock-We've already seen quite a bit. But, I feel like they could explore a little more about her mother and the past Queens of Amazon Lily. She could show up again, in one of those, if they're important. It's more likely that stuff would be answered in a SBS, though.
Tashigi-It's unlikely, but we could get a flashback that explains if there's any actual link between her and Kuina. It'll probably just be in a SBS, if anything at all, but it's not impossible.