Hunter x Hunter is unique in that you get to choose and develop what kind of ability you want. But Gon doesn't really display that in his Jajanken. Killua's electricity powers show it a little more, but the two protagonists still don't seem to have the kinds of powers that fit them as well as Kurapika's chain ability or Hisoka's Bungee Gum.

First, let's look at Gon. His power has the following requirements:

It needs to have an impressive and visible powerup that allows him to defeat Nerferpitou during the Chimera Ant Arc with ease (Any mind control ability wouldn't work, since it's hard to power up that sort of thing and Pitou can puppet herself out of any mind control. Defensive abilities also wouldn't fit here.)

It needs to fit within the Enhancer category, but it can take on aspects of Transmutation and Emission. Netero's Bodhisattva is also in the Enhancer type, so the category is more broad than you think.

It needs to be something that allows him to defeat Genthru during the Greed Island arc with some difficulty.

It needs to be simple enough that a 12-year-old can come up with it (An ability that redirects the trajectory of satellites launched in the air to crash into opponents while the user corrects for air resistance, gravity, and weather would not be it)

It needs to let him be a crucial part of winning the volleyball game against Razor (This is the most restrictive requirement)

I think the power to impart objects with his aura to store energy and forces would fit. This is something similar to Sale's Kraft Work from Jojo's Bizarre Adventure. He can use his aura to empower objects to resist heavy blows, defy gravity, and not move when colliding with other objects. But he only holds it for so long, and when the power is canceled, all the forces that the object has experienced rush back at once, causing it to immediately sink to the ground from accumulated gravity, or suddenly rush off in the direction of where it was hit.

This fits with Enhancement and Transmutation, since he is enhancing objects with his power. He can even enhance himself to keep going and resist damage at the cost of collapsing after the forces have accumulated. It allows him to repeatedly punch the volleyball in Killua's hands to store up kinetic energy and blast directly at Razor. And against Neferpitou, he can power his aura up not with forces from the environment, but with his life force, costing him his power.

Killua's power also has some requirements:

It lets him defeat the Ortho siblings residing in their lake.

It fits with his role as an assassin. (Shooting giant lasers like his dad or raining down energy from the sky like his grandpa is not it. Seriously, why do they have the least suitable assassination abilities ever, unlike Illumi and Kalluto?)

It fits with his power as a transmuter.

I think the ability to transform his aura into a variety gases is an appropriate power. It's similar to how Killua realized his electricity power, in that he himself is immune to the effects of his power due to his learned resistance to electricity, or in this case poison.

The use of toxic gases also fits his role as an assassin, as he can quietly turn the air you're breathing into hydrogen cyanide to kill you silently, conceal himself in colored gases, and use varying densities of gas to muffle or change his sound.

It allows him to defeat the Ortho siblings. He just needs to realize that they're residing in the lake, jump in, and fill it with hydrogen fluoride gas. Or use his own aura to supply himself with oxygen, break down their door, and kill them directly.