Theory on Loki

Forgive me if this has been thought of before, but I'm guessing Loki's unspecified Devil Fruit is the Gomu Gomu no Mi, whereas he thinks he ate the Hito Hito no Mi, Model: Nika.

Loki believed he was killing his father for the Hito Hito no Mi, Model: Nika, which would make him the Sun God Nika. He knew the legend of Joy Boy and Nika, and he was willing to kill his own father for the chance to be the prophesized return of Joy Boy. He refers to himself as the Sun God of Elbaph because he still believes that he consumed the Hito Hito no Mi, Model: Nika and currently has Nika's powers.

But we know that he didn't consume the Hito Hito no Mi, Model: Nika, because Luffy did. Loki killed his father to consume what turned out to be the Gomu Gomu no Mi, which we originally thought Luffy ate. Loki isn't a Sun God like he thinks, he's really he's just a giant with rubber powers. The powers are similar to Nika's, and will be especially formidable when wielded by a giant, but Loki is more like a giant cosplaying as a god than a real god.

The end of this story is Luffy revealing his powers and thrashing the false god Loki. This was foreshadowed a bit earlier in this arc, in that Road, the false "Sun God" of his own doll world, was struck down and revealed as a fraud. I am guessing that Loki's Gomu Gomu no Mi will let him expand to a similar size compared to Elbaph as Road was to his doll world, effectively becoming a giant among giants. This is maybe even what Laugh Tale was about, Roger's pirates finding out that the Gomu Gomu no Mi and the Hito Hito no Mi, Model: Nika got confused for one another, and Loki had committed patricide for an ordinary Devil Fruit.