If you're in tech, you've probably used an Atlassian product (e.g., JIRA, Confluence, Bitbucket).
Make an Atlassian add-on!
It's been a (mostly) solo gig for me so far. Two months ago my own add-on happened to do $8K in sales within a 30-day window. That put me around the 150th position on this ranking (add-ons ranked by last 30 days of sales):