The BDI agent model comprises a simple but efficient folk psychological framework of mentalistic notions usable for modeling rational agent behaviour. Nevertheless, despite its usefulness it is also a popular subject for extensions that try to improve the model in certain uncovered aspects such as emotions or norms. On the architectural level the BDI model is typically represented by an abstract BDI interpreter, which implements the fixed BDI reasoning cycle. In this paper it is argued that a fixed cycle has certain inherent drawbacks and that a transition towards a flexible agenda approach based on BDI meta-actions leads to a design open for extensions in many respects because new meta-actions can be easily integrated into the architecture on demand. To prove the validity of the approach, it is shown how the extensibility can be exploited to integrate concrete new aspects of increasing complexity into the model. They range from a simple mechanism for updating beliefs to a complex goal deliberation strategy and demand only slight modifi- cations at well-defined extension points of the architecture. The new architecture as well as the presented extensions have been realized within the open source Jadex BDI reasoning engine.