Looking at https://firebase.google.com/docs/rules/basics, would it be practical to have a "simple security mode" where you can only select from preset security rule templates? (like "Content-owner only" access or "Attribute-based and Role-based" access from the article) Do most apps need really custom rules or they tend to follow similar patterns that would be covered by templates?

A big problem with writing security rules is that almost any mistake is going to be a security problem so you really don't want to touch it if you don't have to. It's also really obvious when the security rules are locked down too much because your app won't function, but really non-obvious when the security rules are too open unless you probe for too much access.

Related idea: force the dev to write test case examples for each security rule where the security rule will deny access.