Trying to put together a complete list of all potential callbacks:

  Class creation                   Class#inherited (sort-of), or alias #new.
  Class reopening/closing
  Module creation
  Module reopening/closing
  Method definition                Object#method_added
  Singelton-Method definition      Object#singleton_method_added
  Constant assignment*
  Global variable assignment       Kernel#trace_var (sort_of)
  Class variable assignment
  Instance variable assignment
  Local variable assignment
  Module inclusion                 Module#included
  Module extension                 Module#extend_object

  *Constant assignment may include class and module creation.

See any anything I'm missing?

Thanks,
T.