This paper addresses the challenges and opportunities offered by post-CMOS devices in terms of new functionality and ultra low power. It focuses on the illustration of: (i) the quest for the new electronic abrupt switches (tunnel-FET and IMOS/PIMOS) and (ii) the quest for new functionality that can be offered by some non-pure electronic functions (as nano-mechanical integrated functions offered by Suspended Gate - FET). It is shown that beneficial interactions between the Beyond CMOS and More-than-Moore domains should be considered in the post-CMOS era. A system design perspective with particular attention to the true benefits at circuit and system levels and the need for accompanying the technology and device effort by adapted design and/or new system architectures, is discussed and illustrated with some examples concerning power management, digital and analog applications.