He could be important to Dragon's backstory
In Manga, his Face was hidden with a Speech Bubble, meaning in Anime We should have the Full Scene, yet They even hid his Face in Anime.
Oda did it Twice actually, and even showed him Sleeping in a Third Panel, but only Half his Face shown, and He didn't look Goofy at all
(Such as literally every Irrelevant King We've seen)
I have a Feeling He somehow became a Celestial Dragon
And Oda wouldn't create this Sub-Plot just for sake of Introducing another Sadistic CD
If He aimed to become a CD & Succeeded, then why Stop there? This guy might be aiming to become a Gorosei or even sit on Empty Throne
And He could be created just to have the Role of a Traitor among CDs & an Ally of another Major Villain
If He wants to Sit on Empty Throne, He needs Support of a Major Faction/Army & i believe it could be none other than Akainu
Akainu & Former King of Goa could be Secret Allies who are Planning to take over World Government
After all, Akainu have Absolute Justice yet He seems to hate CDs (Unlike Lucci or Ryo for example)
And this whole Idea of Burning Everything Down fits him as a Magma/Volcano Man & Oda used it before in Film Z with Zephyr
It also creates another Connection with Sabo, who needs to take Revenge against Former King of Goa, CD who Shot him & Akainu