Katy will never work with him again. Capital would never allow that.
Really don't see who would work with him tbh. His name is attached to such a strong negative image that it would tarnish any song's change of becoming a hit now.
Not to mention it seems like he's been having trouble keeping up with the shifting Top 40 musical landscape. He's just about finished it seems
I didn't think that Katy would work with him, even if he wanted to be a ghost-writer. That would put her reputation in danger, when she can just work with Max Martin and avoid the mess.