Jason is possessed by Vecna, but rather than just kill him Vecna decides to play with him. He knows Jason has a hero worship thing about Steve, he knows Jason wants to fuck Steve, he knows every one of Jason’s dark twisted fantasies about taking Steve and claiming him and making him his. Vecna conjures the form of Steve and forces Jason to fuck him, having Vecna-Steve ride Jason’s cock while Jason struggles against the forced arousal he feels, trying to deny how good it feels and trying to fight against the pleasure. Maybe Vecna-Steve smiles sweetly and acts soft as if they’re lovers, or maybe struggles and cries due to Jason’s rape fantasies about Steve, or maybe both just to be cruel. The entire time Vecna watches, taunting Jason about how disgusted the real Steve would be if he knew about his desires, how this is as close to Steve as he’ll ever get.

It can end any way the filler likes, I mostly wanna see Jason forced to fuck a fake Steve and having a crisis about it thanks to his enormous lust for the real Steve.