Well, there is a chance that it might NOT actually be a "false" Sun God. It's possible that "Sun God" is just a title in Elbaf. And this current Sun God inherited this title after Nika died. "Sun God" just might be the title of the leader of the village or something.
We see them call a giant rabbit a "Ear God". I doubt they were worshiping the rabbit or thought that it gave ears to humanity or something. So, there's a indication that "God" is just some sort of title. Maybe indicating power or strength or leadership? The most powerful member of a species, maybe? Something like Shinto with nature gods, perhaps?
But, yeah, a "False Sun God" that Luffy needs to dethrone would make for a decent antagonist for this arc. I'm just not 100% sure that's where we're headed with this.
After all, the others do want this Sun God's help with the fire. It's not like they said "We have to take care of this fire before the Sun God finds out". They seem to actually WANT this Sun God's help with stopping the fire. They don't feel the need to hide this or risk his wrath. Which makes me think this guy, whoever he is, might not be a outright villain. Maybe he's still fooling or tricking them, but that doesn't necessarily mean he's a bad guy.
But, oh yeah, Yggdrasil's going to burn down before the end of this arc. That's a thing in Norse Mythology during Ragnarok. And I have very little doubt it'll happen here.
