Kaido's and Big Mom's backstories weren't really that fleshed out. Most of their lives as pirates wasn't covered (the formation of Totland, the Charlotte Family fathers, how did BM found a Three-Eye Tribesman to have Pudding, Lola's arrangement with Loki, the formation of the Beasts Pirates -- the recruitment of Queen and Jack -- and their territorial expansion, Kaido's alliance with DoFlamingo/Caesar, etc.). WB's life is also unknown prior to his stint with Rocks (IMO, it would be nice to see WB's youth years, on the crew we see in his flashback before his death).
Both Sengoku's and Garp's backstories are really obscure too -- it seems that they're top tiers already by the God Valley Incident, but how they got there is unexplained (how was Garp's childhood and why did he look like a pirate hunting for treasures? how and why did Sengoku became an Admiral? And a Fleet Admiral?). Even Xebec seems to be like that too, he was already a top tier 18 years prior to God Valley (what did he do prior to that?). Plus, there are also the HKs'/Gorosei's/Imu's backstories that are completely unknown so far.
I guess we'll never see many of those backgrounds, not even in SBS's (especially for Mihawk, as it seems that Zoro's antagonists are the most obscure of all). That'll be up for our imaginations.