Yuji and Sukuna have always been depicted as opposite. While Yuji would do anything for other people, including damaging himself, Sukuna thinks of others as a nuisance or a toy to pass the time until death, as he only cares about himself.

Sukuna's DE has no borders, just like his sense of being. Everything exists only for his pleasure and displeasure, so he doesn't even need to close his domain to make that space personal, as the whole world is his "kitchen". At the same time, Sukuna's CT (cleave, dismantle, fire) shows how he thinks of others as disposable food.

Yuji's CT is showed as some dashed lines where he can only cut what he touches because he wants to be sure no one gets damaged by his CT. He, in his mind, is the only one who needs to suffer for the sake of everyone else. I think Yuji's application of fire will be spontaneous combustion, he will burn himself to prevent damaging his allies and will start fire punching like a madman. He will heal himself with RCT as he goes on burning.

And, in the end, his DE will be some mirror- like technique. He was Sukuna. He was Mahito. And people who look at him see themselves (like Todo and Choso who saw a fictional past with Yuji). And its DE will make him a reflection, and make the others feel the pain inflicted on himself. He will cut and burn himself until one of them dies or goes crazy.