(possible spoilers)
With the recent chapters and information that has came out, I think it's become fairly obvious that despite everything that's been revealed, there's still much more that needs to be confirmed.
One obvious one is Shank's origin. Clearly he is of world noble blood, but obviously there's more to it than that. Shanks has intentionally been shrouded in mystery, and it seems there is two very clear connections formed with him. The first is the most obvious and the one that's recently been hinted at, and that's his connection to the celestial dragons and God's valley.. and the second one is his connection to those that are in the "D" clan.
Obviously, there's many things to go over with all that, but all I have to say about that is.. clearly he has some kind of deeper goal in mind. But the other obvious point to bring up is.. queen lily. Recently confirmed to be a possible progenitor of "D" clan members, and with her originally being chosen to be a celestial dragon, I think it might be very possible that the "D" members are in fact.. "Fallen angels".. They have all the same "powers" that celestial dragons seem to possess (such as conquerer's haki), but unlike modern day celestial dragons, they face hard ships so actually train themselves to utilize these abilities.
There's two main groups we've seen that have shown utilizing conquerer's haki more than other groups, and that's D clan members and Celestial dragon's that have actually had to take part in warfare. Now as far as what makes Shank's so noteworthy is that not only is he a celestial dragon (from his mother's side, Figarland Garling being such a racist a****le reminds me of a racist father hating that his daughter fell in love with someone of an ethnicity he despises..), but he is also a "D" clan member, with his father being Rocks D. Xebec. (Him being a Figarland is apparently public knowledge, after all, one can't deny ones mother, but he apparently kept his father's identity secret.. and for good reason.)
Honestly, it sounds alot like a forbidden love story (like romeo and juliet), but with Garling learning of what happened and seeing the potential his grandson had, he killed his daughter and tried to keep him "safe" from Rocks, and that started the war on God valley..