Introducing an object factory instead of the use of "new" statements would allow extending core and plugin classes by simple polymorphism and then creating tons of new extension opportunities that go far beyond the current action/filter model (which would remain great and useful) by facilitating the process of writing object-oriented plugins.

In order to stick to the WordPress way, the design patterns to be implemented should be:
* Generic Factory: the class name and the construction arguments would be passed as arguments
* Static Proxy: even if the factory itself would be an object, it would be accessed through a normal and classical function

The Singleton design pattern would also be a natural and useful addition.

I filed a feature request in the Trac bug tracker with a proposed implementation for this new API. To make it short and sweet, the basic services offered would be:
* Register a class: set the class name and the class file, as well as the class it’s optionally a substitute of by extension
* Register an object: set the unique name and polymorphic class of a singleton that may be called later
* Get a registered object: provide with the named singleton created from the right registered class
* Create an object: provide with a new object created from the right registered class

Such an object-oriented foundation might also be the first stone of a fully object-oriented WordPress in the future, who knows...