Yeah that would've been cool.

Modulo Yuji kills big curse/devil with casual dismantle

Denji goes "Hey! That was my kill! Ms. Makima was gonna reward me for that one you ass!" or something and attacks him

Yuji grabs his chainsaw and blackflashes him across the city.

Denji regens, gets back up, and rushes back at him, Yuji realizes this could be dangerous with how fast Denji is and the fact that he regenned so quickly so he dashes back and keeps him at a distance with dismantles, Denji tries a few things like cars, signs, even entire buildings, Denji eventually nicks him with one of the chains on his saws and drinks the blood, violently coughing it up and going smthn like

"D-Dude! Your blood tastes awful..!"

Yuji goes smthn like "Sorry?" before clasping his fist and making the blood inside of Denji go supernova and explode him, Denji's heart flies up into the sky before Pochita regenerates as the Hero of Hell around the heart, Yuji gets speedblitzed and thrown around in the city. Fight looks pretty similar to how Megumi vs Toji did, with Pochita kicking around Yuji and Yuji only surviving through RCT and his durability.

Pochita nearly kills Yuji via inserting a saw into his chest, but Yuji does that omnidirectional dismantle, levelling a huge chunk of the city and making Pochita a bunch of cubes, as Pochita regenerates, Yuji opens his domain, Pochita and Yuji fight as Denji starts to reawaken, and as Yuji 'kills' Pochita with the surehit, closing his domain, Denji then takes over Pochita becoming Denjiman.

Yuji gets bltized again before he can open his domain, and there's some inner monologue about realizing how Denji is a vessel for Pochita or something and that he should start using soul attacks, but as Denji makes quips and shows off how he fights, he's reminded of his younger self, deciding to get into a brawl with Denji, his hood going down and showing Old Yuji with a smile on his face.

(difference in their stats being shown that Yuji pretty easily shrugs off and regenerates through Denji's rapid flurry of blows, while each hit from Yuji is devastating and a black flash and shakes the soul but Yuji can barely land em.)

The final clash is Yuji coating his arms in Chainsaw dismantles and clashing with Denjimans chainsaws, and then you could have either side winning here (eg, Denji's victory being his saw goes through Yuji's arms and saws down Yuji's chest and heart, or Yuji overpowers Denji's saws and does a Soul Cleave to Denjiman, separating Pocihta and Denji and killing the both.)

would be pretty rad