TYPO3 Core

The majority of modern Frameworks, PHP or not, that use the MVC concept, are using out of the box extendibility. Either with events or other methods. Extending a domain model and having the changes/extends directly there without having to do anything else, could come in handy, specially when multiple extensions, extend one extension and the domain model has to be extended multiple times and have all the extends available.

Here is an example: https://extensions.typo3.org/extension/extender

We should start making the extendibility of other extensions easier and finally get rid of the XClasses.