Also in SHs Crew, only Luffy & Zoro & Usopp (He lied about it) Confirmed to have CoC
And there is something Unique that all these Three done in entire Story, which is a Clear Foreshadowing
Only Luffy, Zoro & Usopp ate Dragon Meat
And then Luffy said "I now have Dragon Powers"
Robin for example didn't Eat & Oda made sure to show this Detail (And Confirm that all Three others Ate it)
Nika's Dance, Flaming Hair, Personality, CDs Love for Western Dragons, Luffy's Obsession with Meat, this Punk Hazard Hint ... etc
All indicate that Nika was simply a "Dragon Hunter" & Dr. Hiruluk was a Mini-Version of his Story
So just like Drum Island had Sickness Problem, Nika lived in a Kingdom that had Dragons Problem
And just like Villain Wapol took 20 Doctors to himself, Villain during Nika's Time also took 20 People who were capable of Fighting Dragons
Does 20 ring a Bell? Obviously Founders of WG, and OG CoC Users
This is clearly inspired from Order of The Dragon which also had 21 Founding Members (During Drum Island Arc, Kureha was Doctor 21 who refused to join Wapol), so They are Kings who gained "Dragon Power" which is CoC (This is also why Garp named his Son Dragon, cuz CDs are only Descendants of OG CoC Users, but Garp is among those who Inherited their Power, so his Son have more right to be called a Dragon than CDs. It's also why Trebol & Co. believed Doffy should Rule the World, cuz He is Both CD & CoC User, that's why They instantly bowed after He demonstrated CoC)
Also Oda isn't Highlighting Ryuma's Dragon Slaying Feat just to Hype his Power, He actually keeps reminding you that a Dragon survived Void Century & lived during Ryuma's Time (Probably Last Living Dragon, that's why the Story is so Famous), Robin for example didn't believe Dragons could Exist (Meaning that in 800 Years, there was no Sign of Dragons, making Ryuma's Dragon the Only One)
This indicates that it was possible for another CoC User to be born during Ryuma's Time (Not Ryuma)
Making the Total of those who Inherited Dragon Powers 22
This means there are 22 Families who have the Right to sit on Empty Throne
And also 22 CoC Users who are more Deserving of it