A Domain Expansion is achieved through an overwhelming sense of self and represents the ultimate manifestation of a sorcerer’s ego. Sukuna’s divine feat of achieving an Open Barrier is representative of his ability to project his sense of self onto the world. Megumi’s incomplete domain represents his half-hearted sense of ego, his overwhelming sense of self isnt complete because he’s a selfless person, and its shown via his Domain buffing his Shikigami and technique. He has an emotional connection to his Shikigami and fights WITH them. Yuji has no overwhelming sense of self. His Domain was achieved through an overwhelming sense of selflessness. His Innate Domain manifests as his home town, and requires manual activation of his sure hit because he believes in giving others a chance at redemption. He is compassionate and we literally get shown a buddha statue and his mudra is that of a boddhisatva who delays his own enlightenment to guide others to enlightenment. His Domain doesnt need a name because it goes against the very nature of what a Domain is. And this is expanded on in Modulo with his self-isolation and “old soldiers never die, they fade away”. He has no overwhelming sense of self, he echoes Gojo’s wish to be left behind because thats the role he gave himself. Its also why he vows to fully train the next generation, track down every HR lineage and turn himself into a Curses Object. His role is to guide others. Thus his Domain doesnt require a name.