Perhaps what you are looking for is a more powerful "defining" semantics?

newtype MyFoo = Foo defining (Foo(..)) -- all class instances that Foo has are delegated through from MyFoo

Matthew

not sure if this is what you are thinking at, but everytime I wrap a
type Foo in a newtype MyFoo to define my own instances (or just for more
expressiveness code), I wish I had a way to tell the compiler:
„Please define function myfoo to be the same as foo, with all occurences
of Foo in its type signature replaced by MyFoo.“