The only path left is female humiliation. unless they do something like Felix and the other female that help Sophia down that road get together.
I am not a fan of humiliation so i'd rather see them explore more. It would be fun to see Zach push her to explore her boundaries. have her do some exhibition, tease her students, go to a glory hole, a nude beach, maybe share? there's a lot you can do without humiliation.
