Well from the beginning it can just be community release workflows. Which will be well opinionated. It can be hosted on github, anyone can make request change for something.
It should also be versioned, for specific versions of XCode, Android Studio, since they will be breaking changes.
Later on I think, share workflow button can be added, so anyone can export their workflows, but this might be more tricky, since people should cleanup their workflows and create readme with How to setup workflow, all env variables and so on.
So at the end, I think just create public workflows for major use cases and allow others to contribute.
I like coda.io templates: