Blackbeard was a Slave in God Valley
I am more and more convinced that Blackbeard was a slave in God Valley, exactly like Kuma. This would create a parallelism between Shanks (as a heir of World Nobles) and Blackbeard (as a heir of their slaves).
Moreover, Blackbeard (who was a baby at that time) somehow he was saved from being killed and was told / he found out that slaves were saved by Rocks D. Xebec attacking God Valley. He understood Rocks as being his saviour, as the one thanks to the whom slaves were freed. So that Rocks became his idol.
Also, Buccaneers believes in Nika, who they believed to be the one who “save people from suffering and grant them their freedome”. What if Blackbeard misunderstood Rocks’ role in God Valley and considers Rocks as being Nika?
Finally, Oda shows Blackbeard crying in front of the moon. What if the moon here represents Garling Figerland, the one of killed his family in God Valley? This would create further grudge between Blackbeard and Shanks.
*Theory by L7Z7Z