Integration and increased uses of unoccupied aerial systems (UAS) challenge current airspace operation. Rather than centralised airspace management (which is rapidly reaching capacity limits), those vehicles need to collaborate safely and efficiently. However, the vehicles differ significantly with respect to capabilities, carried equipment, and certification requirements. The main focus of this paper is how to determine a safe level of interaction in a heterogeneous network, where not all vehicles are (equally) trustworthy, but cooperation is required for many different reasons (e.g., collision avoidance, implementation of collaborative tasks). Consequently, this paper presents the main research challenges deriving from integrating UASs in a shared airspace, with a focus on the demanding scenario of urban air mobility. Specific use cases are described to highlight the main challenges and requirements for a security architecture. Furthermore, a roadmap is presented towards addressing the main challenges: trust estimation, interaction adaptation, controlled information sharing, and continuous monitoring and adaptation.