Since MadMol had many things covered, I'll give few other fantasies of mine:
Your mother is a hard religious woman (muslim, jw, mormon, catholic, etc), and she taught you that all sex is evil, to the point of traumatizing you, and every time you think about sex you fear that you'll go to hell. Now, imagine this woman is having extramarital sex with your bully. throwing away everything she taught you. I bet you'd be having a faith crisis (and probably nervous breakdown).
Another fantasy is incest. Imagine that your bully misses with your mom's brain. She doesn't see you as her son anymore. First she hates you, and slowly she thinks you're hot of uses you get her rocks off.
Another one is abandonment, not only emotional, but goes further to virtual disowning. She stops talking to you, then feeding you and clothing you, or even kicking you out in the streets.
Another one is that the bullying never ends. Imagine that twenty years passed and your bully still humiliating you. He has a good life now but you're still a loser. He has a wife and family and still fucks your mom for the sake of humiliating you.