How about Son of Psycho?
How about they do another sequel? Norman is long dead. His son is fully grown, and yes, he has the "psycho seed" just like Norman was afraid of.
I really don't know how it would all play out, but I did think of one detail that could make for a really crazy idea.
We're all used to Norman talking to his mother's dead body, well what if Norman's son did the same thing, but instead it was FATHER, and talked to Norman's dead body? So they could get someone who could mock the voices of anthony perkins, telling the boy to kill.
This time around, the boy doesn't dress up as his father to kill, however. Unlike Norman dressing up as mother, the son of psycho kills on his own, but still talks to the corpse of Norman. And Norman tells him to kill, as well.
At the end I figured the Psycho son could die, be shot by police. Thus forever, and finally, truly ending the Psycho legacy, and killing the seed.