Ask HN: Less-saturated platforms for premium plugin development?

What kind of less-known platforms do you know of for developing interesting little paid plugins?

I'm thinking of things like Wix apps (http://www.wix.com/app-market/main) or Slack bots maybe a plugin for another existing SaaS and things of that sort.

I'm interested in making a tiny useful plugin or something and maybe earning the cost of my internet and coffee per month.

The bar is too high for things like Wordpress/Android/iOS and so on and the market is saturated. So I was hoping to find a little under-served niche/slice of the market where the bar is lower.