I am now 100% in believe this ends with Denji yelling "I'm yours!" in a conversation that mirrors a marriage proposal. Then Asa-Yoru crying/smiling as she calls out "Chainsaw Man Chainsaw" and we get full War Devil with a sick ass chainsaw weapon where Denji can talk to her in that form too.
As Asa-Yoru cut into Falling Devil with every slash, Chainsaw Man's mouth at the end of the blades is eating the chunks sputtering off FD's body to erase her by the end of it.