I think these are all equally good. The Impl's name should never need to be referred to, so ultimately the choice should be based on what's easiest to read.

EDIT: actually, your examples wouldn't work as-is, since it would result in circular references (something Python isn't well-equipped to handle). Your target names would need to be strings instead of the actual class.

EDIT2: I have updated the code to allow for another syntax:

    class AnyImplNameIWant(MyTrait, metaclass=Impl, target="MyStruct"):
        ...

It would certainly be nicer if `MyStruct` didn't need to be a string of course. I really think these options are mostly down to taste, so I'll just have to trust that this alternative reads sufficiently more nicely.