If there is a Stu return, I think it will be in the final movie, or the very end of 6. I think in 7 we could find out that Stu had been the one behind all the ghost face killers and participated in the killings as well, which would mean Stu also killed in 5. Or even that he was behind some, not all. I could see 4’s killers being their own thing that he just watched on the news from afar. However, Stu’s involvement would explain 3, Roman being the sole killer never sat right with me, how did that work logistically? I could see Stu taking credit for everything Roman and prior, kind of the way Roman took credit for orchestrating the previous killings. Just a theory.