mixin template Bar()
{
    override void func() { writeln("Bar.func()"); }
}

class Foo
{
    void func() { writeln("Foo.func()"); }
}

void test()
{
    Foo b = new Foo();
    b.func();      // calls Foo.func()

    b = new Foo();
    mixin(b, Bar());
    b.func();      // calls Bar.func()
}

It's crazy idea?