Illumi's Needle People ability, combined with the Sharingan, would create an army of perfectly controlled, terrifying puppets. His manipulation Nen would reach new, horrifying heights.
Ging's unparalleled talent for copying Nen abilities would make him a Sharingan master in moments. He could perfectly replicate any technique after just one observation.
Kurapika's Emperor Time already boosts his Nen to terrifying levels. With the Sharingan, his versatile chains would become an unstoppable, adaptable weapon against any foe.
Hisoka's love for mind games and trap-setting would be amplified by the Sharingan. Bungee Gum combined with precognition would make him an unpredictable, deadly illusionist.
Chrollo's Skill Hunter, which steals Nen abilities, is a natural fit for the Sharingan. Its observational power would make him an even more formidable technique thief.
Tserriednich's Parallel Future, seeing ten seconds ahead, combined with the Sharingan's insight, would create an inescapable, predictive combat loop. He'd be virtually untouchable.
Meruem's unparalleled genius and rapid learning ability would make the Sharingan terrifyingly potent. He would master its full potential and all copied techniques instantly.