Going further: I would like to have an attribute that declares a trait to be dyn-capable. Something like:
#[dyn_capable]
trait Foo { ... }
It would then be a compilation error if that is not the case. We would lint (warn) if you do dyn Foo and Foo is not declared as dyn_capable.
The purpose of dyn_capable is then to declare your intention that Foo should be usable as an object, which implies accepting limitations on its design.