There is something weird from the theft of Ryuma. It happened 23 years ago, before Moria was broken by Kaido, or he had recruited Hogback to make zombies.
We don't even know if he had the shadow fruit back then.
I'm the first person that wants to see Moriah back again in the story, but right now, without confirmation of him being the thief of Ryuma's body, the link back to the main plot becomes shaky, as he would not know how to get into Wano, and has no reason besides an unknown number of years of vengance that he didn't consumate in that time, and is right now in a worse state than when he started.
Maybe he was already collecting corpses before his fruit? Maybe he already had the fruit, and was starting to experiment on making zombies, and decided to wait with the good bodies until he got someone like Hogback? Maybe there is another reason?
We are going to go through Ryuma's story, through Zoro and Brook (Zombies were made of a body and a shadow, that created a new personality that was almost always the shadow's, only exceptions were Cindry, who was shaken to the core by Chopper for a second, and Ryuma who acted more Ryuma-ish than Brook-ish during the battle, Lola keept some memories from Warthog Lola, some cross contamination Ryuma->Brook's Shadow->Brook could have happened)
Thinking about it, Brook, did he know about the warlords? I am under the impression that the warlord system is a product of the great age of pirates, but if Brook knew about them.. I'll have to reread thriller bark.