Ask HN: Looking for a 3rd-party ecosystems as a consultant

Hello HN-ers!

I've been thinking to invest my spare time to build business (side-income ish level) around 3rd-party software/service and am looking for a growing niche SaaS/Software/Platform ecosystem that is small enough for a single person to tackle.

An example would be a Shopify developer (Salesforce ecosystem is too big and Wordpress ecosystem is too crowded).

I'm a software developer with some CSS skill (I can hack it but by no means I'm an expert) thus prefer more software development than theme development (but if that's the way to go, I don't mind :)).

Any recommendation? (Oodo ERP... SugarCRM or something...?)

Thanks!