I am thinking about having some hooks for the custom logic. I like the entity listeners/subscribers from typeORM: https://orkhan.gitbook.io/typeorm/docs/listeners-and-subscri... but I the end I guess we would probably need some functions stored somewhere. Not sure how to implement that though

There's probably some low-hanging concepts of rules, like A can have X of B. I guess if you work out what four or five of the most common rules are you could build just for them and have the rules in the YAML, as they'll be enough for the tools using backends like this.

Limits would be a good name I think for that rule.