I just looked at this implementation. With my limited architecture skill / experience, I could say it is right or wrong. However, I do have some questions below:

1. How do you plan to make this compatible with Controller implement in 4.0-dev branch? In 4.0-dev, controller constructor has 3 parameters, $config, $app, $input. Will you pass $factory as the fourth parameter? So if you keep this implement for Joomla 3.8.x (only 2 parameters), then there would be conflict with 4.0?

Assume you pass $factory as the fourth parameter, $factory keep an instance of application, and we also has $app as parameter of controller parameter, so basically, there are 2 apps object passed to controller constructor, seems it doesn't right.

2. About optional parameters in factory methods

Look at createModel method

public function createModel($name, $prefix = '', array $config = array())

Can $prefix really be an optional parameter? I think the only parameter should be optional is $config. If someone calls this method without proper $prefix parameter, it won't work as expected (the system could never create a model with only $name parameter)

In current legacy controller class, $prefix is marked as optional parameter because it is a protected method. The only way to actual get model in controller is via getModel method and that method does calculates default $prefix if needed before calling createModel method.

The same question is applied for createView, createTable methods.

3. Is it logic that a MVC factory doesn't has method to create controller?

1. This needs to be backported to 3.8. But yes it will become then the fourth argument.
2. In legacy, the prefix is optional for JModel::getInstance, so I guess it is fine then.
3. Creating the controller is done in the dispatcher.

Let's fix conflicts here. I think we probably do want to end up moving controller creation into the MVC factory too. but this is a good starting point.

@joomdonation is taking over the namespacing part so I'm out.