I think Sun God Nika is probably a diety which existed a very very long time ago.
Joy Boy inherited Sun God Nika's will and the former user of gomu-gomu, a.k.a the first Nika.
Luffy is the second Nika.
There probably will be another God which is going to be the counterpart to Sun God Nika.
Emo(Imu) the Sad Girl have inherited this evil God's will.
:cheers: