Great topic. It’s really hard to collaborate on Anvil projects. We’ve developed some reusable stuff in several projects that we’d love to share, including some interesting UI components, and bindings to use Google Firebase services from the client, but we couldn’t find an easy way to publish them, while providing the ability to manage versions and updates. We can’t even manage them internally very well. I really admire @owen.campbell for publishing Anvil Extras. 
For me the core capability is the machinery to package and publish an app containing code and/or components in such a way that it can be imported into any other Anvil app (in any account). Anvil already has some of the machinery to support this: I can attach another of my own Apps as a dependency, and import its code. So what’s missing?
The main thing seems to be the ability to attach someone else’s app as a dependency (without cloning it, which is useless in the long run): There is no shared Anvil app repository. If we could solve that, I think we’d have something. Versioning and immutability (so published packages can never change) would be key features.
I’m not sure such a repository needs to be curated, or support purchases. Those are certainly interesting ideas, but most successful code repositories don’t do either of those things.