(defclass my-class (standard-class) ())

(defmethod initialize-instance :around ((class my-class) &key)
  (let ((instance (call-next-method)))
    (format t "I got an instance: ~S~%" instance)
    instance))

(defmethod c2mop:validate-superclass ((c my-class) (s standard-class)) t)



CL-USER> (defclass foo () () 
           (:metaclass my-class))
; I got an instance: #<MY-CLASS COMMON-LISP-USER::FOO {10106063D3}>
#<MY-CLASS COMMON-LISP-USER::FOO>