This is great and I'm definitely in favor. I am a little concerned about the potential for confusion in the checkFinaleReadinessOpenCoded example with the 'any Costume' isn't necessarily the same type as 'any Costume' error.
What if, in the same way that implicitly-unwrapped-optional types (Foo!) become simply optional types (Foo?) during type inference, any P became some P during type inference? That is, in the declaration of costume in costumes as well as in all the further examples, if a variable's type isn't explicit, it is the opened some P type rather than staying any P? In this example, that makes things even more ergonomic. Are there other situations I'm not thinking of where it would be problematic?