Some good old speculation.
Domain expansion is considered to be the pinnacle of sorcery in Jujutsu Kaisen. It allows the sorcerer to create a self-contained domain, in which they have absolute control and can manipulate the environment and the rules within it.
Only the most skilled sorcerers are able to use this technique because it requires a high level of control over one's cursed energy. After creating a domain, the sorcerer becomes its ruler, which leads to a significant increase in their powers.
Although being quite powerful and a grade one sorcerer, Nanami Kento doesn't possess the technique of domain expansion. And even though we can't know for sure what it would look like, fans have got some theories.
Domain expansion is said to guarantee any cursed techniques used within the domain to hit their target. Nanami's technique is built on dividing his target with lines, thus creating a weak spot. It's called the Ratio Technique because the spot is created at the ratio point of seven to three. This helps him deal greater damage and finish his opponents faster.
Fans suggest that if Nanami possessed the domain expansion technique, it would raise the number of times the division occurs to much higher levels, which will allow the sorcerer to hit every inch of his enemy's body with cursed energy.
Moreover, some fans add that it would be really awesome if every single collision would count as a ratio, so even touching air or the ground would result in Nanami dealing severe damage. This way the sorcerer's powers will be brought to another level, and if an opponent gets stuck in his domain, their days are pretty much over.
The only thing that can stand in the way of Nanami's victory then is the healing abilities of his enemy. If they're quite strong, they might save the creature unlucky enough to battle the sorcerer. But fans suggest that even healing could result in collisions, thus continuing to hit its owner.
There are some doubts about this technique being visually portrayed but fans agree that these abilities would be really astonishing for Nanami to have. Going back to the visual aspect of the technique, fans mention Nanami's first fight with Mahito and suggest that the sorcerer will continue seeing his opponents with scales on their bodies.
It's a shame that we will never see what a domain expansion by Nanami could look like in the anime, but having a good imagination is a cool way to approach this question.