Look on indiehackers.com hundreds of businesses with revenue for inspiration.

Based on this broadly I think sell something that is highly valued. Which much of the time means selling stuff to marketers in some fashion or helping people directly sell more.

Another is anything that can help a team be better. A team can easily cost a million a year in salary.

Bad things are things people don’t care enough about or are freely available or the end user is poor. I think generally budget apps would be bad (user is trying hard to save at the buying point!).