I think it's become clear that many of the Straw Hat's pasts aren't told yet. Luffy's past was first to be further fleshed out in the Sabo flash back and recently it seems that Sanji and Brook will be next. Sanji's past is obviously going to be fleshed out far more with all the Vinsmoke history, however Brook's silence is very telling, I think his past will tie into the vinsmokes and we may get insight into his childhood, how he got his scar, what happened when he lead a battle convoy and when he ate the revive fruit. Other straw hats also seem to have unclear pasts like Franky's pirate parents, Nami being born in a war torn place and Zoro's life before coming to the dojo. I think Chopper, Usopp, Robin and Luffy's stories are done though. We may also see how Jinbei met tiger or how he got his scar when he joins.
Another pattern I've noticed is villains, pre-time skip they had no history for the most part and were just evil however since the skip we have had flashbacks for all the major antagonists and we even had one that made Arlong more sympathetic.
I think this trend will continue and we will get the past of Crocodile, Eneru, Lucci and Moriah. I do think all these bad guys have a story to tell, for example many fans think that crocodile was once a female revolutionary explaining his knowledge of Pluton and links to Ivankov.
What do you guy's think are we gonna see everyone get more flash backs going forward and what secrets would you like to see explained in such follow up flash backs?