Which markets do you feel are missing? Your choice must be technologically feasible and not obviously ridiculous from the cost side.
Hooters Air is going out of business, but on the other hand the works of Brazilian guitarist Bola Sete are now widely available. Markets in pre-ordered parking spaces are underway. So I will nominate "simply paying the tiny but time-consuming library fines of people ahead of you in line (why don’t I?)."
List your wish in the comments, or tell us why someone else’s pick already exists.
Addendum: Yes a toilet lock is already on the market.
Second addendum: It is too easy to name illegal markets, instead try to spot entrepreneurial opportunities.