An area I think is overlooked is selling templates. If you've been a freelancer there's no doubt you would have used a template before whether for invoicing, graphic design, or any other part of the business.
Here are some platforms you can make money selling templates. If I'm missing any feel free to comment below.
No Code
Design
3D Development
Music
Web Development
Photography
You can sell on flurly too! For no monthly fee and only 1% take rate
WeCommerce is an interesting publicly traded company in this space - they're a c.$500m company that do Shopify teams and apps.
Andrew Wilkinson of Tiny is one of the brains behind the operation, and they seem to have big plans to become THE major player in the Shopify ecosystem in the next decade.