It is a key challenge and obligation for future networks to deliver high-quality wireless for enabling novel services and application to flourish for the benefit of humans on a personal level, for society as a whole, as well as for business and innovation. The dynamics, as well as the heterogeneity of the foreseen 6G system in terms of communication technologies, frequencies used, management frameworks, etc. make the need for flexibility of utmost importance. Although several steps are being made to this direction, flexible networks, capable of dynamically adapting to changes, (re)configuring themselves, exploiting highly dynamic resources in terms of availability in a three- dimensional network space is still a challenge from different perspectives. This paper identifies the key challenges, technical enablers, as well as current efforts towards the realization of the 3D, flexible 6G “network of networks” concept.