I actually have a fun hobby of dreaming about unethical-but-legal startup ideas. Then again, whether something is unethical depends on how you define your own ethics, or alternatively, invent some self-justifications.

Munchkin ideas are often unethical but legal. This is because a Munchkin is by definition a way of following the letter of the rules while violating unspoken convention. In some cases, ethics, in contrast to law, means precisely that one follows uncodified rules of behavior.