XMM-Newton is capable of making a transformational advance in our understanding of how luminous accreting black holes work, by dedicating about 10% of future observing time to long observations, of order Megaseconds, to X-ray variable Active Galactic Nuclei (AGN) research. This would enable reverberation studies, already a commonplace feature of AGN, to proceed to the next level and follow the behaviour o f the powerful dynamic corona. Such a dedicated legacy programme can only be carried out with XMM-Newton.