I've been getting fed up with the question of what methods to put in a class which helper methods should be elsewhere. Mixins and extension methods are two ways to address this topic. They have their pros and cons.
But I'm becoming more and more a fan of simple, top-level functions. Wither OOP. Everything old is new again for me, and each function can be an island, especially with multiple dispatch.
Well, there's that question of how to control data access and abstraction of types. I don't have it fully worked out yet, but I'm hoping to find a nice solution that keeps things as simple as possible.