The name Nika isn’t derived from the sound effect of laughter but is derived from the archaic Kanji used for counting fruit: 果 pronounced as Ka (the usage is random#果 ). So if you wanted to for example, count 2 fruits, you would say 二果: Nika: 2 Fruits. I think the Nika fruit itself is a combination of 2 fruits: The Gomu Gomu no mi, a paramecia, and some fruit imagination/creative type fruit.

I think we’ve already seen both fruits represented by some aspects of Luffy usage of the fruit.

The Gomu Gomu no Mi aspect is the pure rubber ,stretching and inflating, represented by base Luffy (or lets call it Gear 1) and Gear 3. The Imagination fruit aspect is what powers the creative side of Luffy’s powers that aren’t intrinsic to the nature of rubber (for example when Kaido says “Rubber shouldn’t be able to do that” in response to Snakeman’s trajectory movement) and is represented by varying degrees of what looks like cloud forming into a Celestial garb of sorts seen initially in Gear 2 as steam and Gear 4.

Gear 5 is the culmination of both aspects of the those fruits fully in sync and used with mastery. What started forming in Gear 2 as steam is a fully formed cloud as if representing a imagination cloud bubble in anime/manga. For the Rubber aspect of Luffy’s powers, he is no longer bounded by the logic before. Being able to stretch and expand his arms to Gear 3 size without having to inflate his hand or becoming malleable when struck by Kaido’s kanabo.

Now this is my theory kind of pushes past that. We all know Blackbeard has eaten 2 fruits. We’ve seen many theories on Blackbeard ability to eat more than one Devil Fruit and the theme surrounding the number 3 with many people have theorized that he would eventually eat a 3rd fruit.

I had a theory based on the childhood picture of Blackbeard crying in front of the crescent moon. Now crescent moon in Japanese is 三日月:Mikazuki. Mika seems kinda similar to Nika. Now I tried to find some connection between Blackbeard and the number 3 and 9 (kind of how Nika is 2 and 9) I couldn’t. However, when we translate 3 fruits using the archaic Kanji for counting fruits we get 三果: Mika: 3 fruits.

In the end, I know the idea of 2 rivals during the void century nicknamed Nika and Mika because they ate more than one fruit which most others aren’t capable of sounds kind of silly but honestly when you consider that void century is archaic and the Kanji for counting fruit 果 is archaic it makes a bit more sense right.

Theory by jumptooconclusions (https://www.reddit.com/user/jumptooconclusions)