Might not be the best use case there, but I often end up wishing there was something like this. I feel like there has to be an existing request for this somewhere. The core problem is that you often have a Type that you want to customize for a particular use case, but you don’t want to mess up the core type that everyone is using. So, you either have to duplicate the type you want to extend, or you end up with domain-specific stuff making the core type messy. So, there is no way to share an encapsulated set of functionality across Types, which could be looked at as a similar concept to Mixins as well.
So, you could have Task, with name, description, assignee, state. Then, you could have a Campaign Task that marketing leverages, which adds a relationship to Campaign. You currently could create the Campaign Task today, but then you need to duplicate each field, action buttons, and rules. By having them completely separate, it takes more work to make sure the core functionality is in sync.
I think the Mixin concept could also be interesting because you could encapsulate a set of fields, rules, etc and apply them to an existing Type. Right now the only way to share functionality like this is via an entire App, which is harder to leverage if you already have something you are using.