If you are looking to enter the job board business here is a good list of what jobs will be hot. A job site needs a strong job market to succeed so stats like these are good to have as you discover which niche you want to target.
To me, I think #1 and #5 and #15 would offer the best opportunity for a new niche job board that targets those roles. There aren’t many specific sites that target those industries. To my knowledge no job board exists today for janitorial/cleaning jobs. That could be an opportunity to explore.