Hmmm.... Nika the Sun God.
Just some rambling, but....
We can infer that he wasn't actually a god, but maybe his abilities gave off the impression that he was.
The only devil fruit I can think of that would give off the impression of a "Sun God" is the Mera Mera no Mi.
Perhaps he was the prior user of the Mera Mera no Mi before Ace, or maybe the original user.
That fruit is now in Sabo's possession, where one of the objectives of the Revolutionaries is to take down the Celestial Dragons, thus they would be freeing the slaves in the process.
This might be the Will of the original Mera Mera no Mi user, which Sabo will fulfill.