Killua as a Transmuter has an 80% proficiency in conjuration yet hasn’t really tapped into it yet, with a repeatedly stated weakness of his being that he is prone to running out of juice while using his transmuted electrical aura.
If he did lean into conjuration, with the already Raijin and Fujin themes of his godspeed and whirlwind, I see him conjuring Tomoe drums which can store charges of lightning available to him during combat, with the number of charges corresponding to the amount of drums he has conjured.
Raijin’s drums are always depicted as connected to his back but for Killua it could really appear as a bracelet or necklace or even collar-thing kinda like the golden ring Zeno has on his outfit.
What would your idea of a conjuration based ability for him look like?