I might not be on the right page here. Are we talking about narratives built around the idea of 'one piece,' like a singular, world-defining artifact or system? In that case, 'Solo Leveling' doesn't really fit—it’s more of a standard gateway and leveling mechanic. For a story where the entire universe seems to orbit a single, incomprehensible system, Will Wight's 'Cradle' series comes closer, especially with the way the Sacred Arts and advancement tiers function as an almost physical law. The 'one piece' there isn't an object, but the path to power itself.
A lot of LitRPG falls into the trap of treating its system as a convenient menu for stats. What makes a 'one piece system' compelling is when it's a fundamental, almost mythological force that characters can't fully grasp. I'd point to stories like 'The Wandering Inn' where the class and level system isn't just a game overlay; it feels ancient, mysterious, and deeply woven into reality's fabric. The system itself becomes the central mystery, the 'one piece' everyone is trying to understand or master, which creates a different kind of tension than just grinding for loot.