Airspace structure is a key factor influencing controller cognitive complexity as it forms a basis for abstractions simplifying controller mental models of air traffic situations. In evaluating the feasibility of new Concepts of Operations (Con-Ops), it is important to consider the effects of changes to the structure of the system and its related impacts on controller cognitive complexity. Examples of key cognitive complexity considerations for future ATC systems are identified by examining three opportunities to modify airspace structure. A part task experiment was used to further investigate the impact of one of those opportunities on controller cognitive complexity, the introduction of time-based control. The hypothesis of structure's impact on controller complexity was supported through an innovative aircraft complexity assessment technique Benefits of time-based control were shown both in terms of controller performance and subjective complexity rating results.