BGE PyAPI support for subclassing any BGE game type from python, scripters define extra functions on gameObjects.

Adding a UI to set the type on startup can be added easily.

# ----
class myPlayer(GameTypes.KX_GameObject):
  def die(self):
    # ... do stuff ...
    self.endObject()

# make an instance
player = myPlayer(gameOb) # gameOb is made invalid now.
player.die()

# ----

One limitation (which could also be an advantage), is making the subclass instance will return that subclass everywhere, you cant have 2 different subclasses of the same BGE data at once.

source/gameengine/GameLogic/SCA_LogicManager.cpp

@@ -307,6 +307,7 @@ void SCA_LogicManager::AddTriggeredController(SCA_IController* controller, SCA_I
 	controller->Activate(m_triggeredControllerSet);
 	// so that the controller knows which sensor has activited it
 	// only needed for python controller
+	// Note that this is safe even if the controller is subclassed.
 	if (controller->GetType() == &SCA_PythonController::Type)
 	{
 		SCA_PythonController* pythonController = (SCA_PythonController*)controller;