5G will have to cope with a high degree of heterogeneity in terms of services, device classes, deployment types, environments and mobility levels. Consequently, diverse and often contradicting Key Performance Indicators (KPIs) need to be supported, such as high capacity/user-rates, low latency, high reliability, ubiquitous coverage, high mobility, massive number of devices and low cost/energy consumption. 4G is not designed to meet such a high degree of heterogeneity efficiently. Moreover, having multiple radio access technologies for multi- service support below 6GHz will be too costly. FANTASTIC-5G is a European project which will develop a new multi-service Air Interface (AI) for below 6 GHz through a modular design. To allow the system to adapt to the anticipated heterogeneity, the pursued properties are: flexibility, scalability, versatility, efficiency and future-proofness. To this end, the project will develop the technical AI components and integrate them into an overall AI framework where adaptation to the above described sources of heterogeneity will be accomplished. The work will also comprise intense validation and system level simulations. The consortium possesses the main stakeholders for innovation and impacting standardization, maintaining Europe at the forefront.