I think Yurui is probably going to be the next "B" -- Darui's "bro" and Killer B's successor as Jinchuriki. Darui will become the new A, and Yurui will become "Bubble B" or something. He does look like he could be related to Atsui and Samui, but I don't think his parentage matters very much.
Shinki was designed to resemble Gaara, so he could possibly be Kankuro's or Gaara's son, if he's related to them at all. He could also be a descendant of the Third Kazekage, which might be interesting as we don't even know the Third Kazekage's name.
I do hope that the Boruto series puts a little less emphasis on each character's particular ancestry. In general I hope the series is, at least for a while, more focused on shinobi life, shinobi missions, and the chunin exams, like a lot of Part I was.