I was thinking more in terms of a full-fledged API SaaS platform that have business logic and data-schemas/storage.

It comes w/ an admin and you just pick and choose what you need, for example:

/users /posts /products /orders /login /emails etc ...

It'd be neat to see a community-driven marketplace to boot, for all types of solutions.

Perfect for SPAs and JAMstacks.

I think maybe Heroku is a better analogy in this scenario?

More like Parse (since bought out by Facebook).

Google App Engine