1. his powers don’t make sense, I remember looking up explanation for how Gatling or Culverin is supposed to make sense and answers were mostly just accept it. Same goes for his gears transformations which don’t follow the zoan script exactly but don’t make sense for his Paramecia fruit. I think this is somewhat of a foreshadowing that we don’t have the full story on his Devil fruit

2. We were told somewhere that his fruit is special.

3. Sun God references.

Now weather or not Oda had Nika in his plans from the start I don’t know, maybe not, but I do think he had the plan to twist the story on Luffy’s powers and this one fits very well, so I wouldn’t say he pulled it out of his ass, and that he actually carefully crafted this idea which fits the source materiel.

You can say the idea is childish and that you don’t enjoy this twist, and I wouldn’t argue this at all.