When I say "OMG IMPORTUNT!" I mean it's knowledge that ties into the storyline, which I doubt will happen. I believe, if we see his parents, they'll be regular people. As with my example of Sanji being from North Blue, it's not important but it's still there.
So then what would be the point of them showing up? My point is that Oda wouldn't throw in another relative of the Straw Hats unless it has SOMETHING to do with either progressing the story or fill in back story. With Sanji or Zoro, it would probably be for the purpose of filling in back story, especially if Sanji's relative is a female.
Luffy's mother or one of his remaining three grandparents, at this point, might serve as little more than fanservice because we've got an awful lot of his family tree already. Seriously, unless his mom is like, second in command of Dragon's rebellion, I don't see why we would need a formal introduction to her in the current storyline.
Zoro's relatives would probably act as filler (in this case, something to add more detail to, not stall the storyline) for his backstory. Hey, maybe there's a special reason he ended up at the dojo.
Nami's family, if they were to be revealed, would be a HUGE plot point because it would take something pretty extreme for her to even have a CLUE who these people are.
Usopp's probably going to run into Yassopp later on. It's strongly suggested he's an only child, so he probably has no brothers and sisters.
Sanji's family would be a good place to explain his chivalry - even if it's just him telling a story about something he saw or did when he was a kid that made him decide hitting girls wasn't cool.
Chopper. . . I think we got enough about his parents.
Robin I have no idea on. Olivia is dead and we think her father is, but it's possible her father's name might be brought up. Don't automatically think it's Roger, either - prior to the 430s, everyone thought it was Gold Roger or Saulo who had fathered one of Luffy's parents. I don't even think Garp ever came up (if someone can prove me wrong on this I'll be highly amused that someone correctly predicted that).
Franky's dad doesn't need to be a big famous pirate, but if he shows up he would probably progress the storyline.
I feel like with the exception of Chopper (unless you add in his DF), there are bits and pieces of the Straw Hat's backstories that are missing, but we have enough extra details to make us question more. So what's going on with Luffy's family anyways? Why did Zoro join the dojo? Who were Nami's birth parents? What made Usopp's mom sick and why did Yassopp leave? Why did Oda feel the need to mention Sanji was from North Blue? Who is Robin's dad? Where did Chopper get his DF? Why was Franky abandoned?
Though we could very well see "The End" without any of these questions being answered, they're still there, and it's possible Oda will address them. Luffy's situation would the exception since we're in the middle of that mess and it's WAY more likely it'll get touched upon.