After the future Legion fused with the past and current Legion, Legion advanced from being a godling with four pathways into becoming the god of telepathy, the god of undying, and the god of killing.
As for being the god of slavery, this wasn't possible right now because they were no longer in possession of the authority of slavery in this current timeline.
At this point, they had yet to acquire the authority of slavery in this timeline, so there was no way that they could become the god of slavery in this timeline.
They advanced to become gods without having to slowly fuse with the authorities in their possession or undergo a lengthy process of advancement and transformation into a god like their future self did, because they have already done all of those things in the future.
