We adopt in this paper a perspective of portfolios of real options, to propose a mixed integer linear programming model for multistage Transmission Network Expansion Planning. The model is then used to analyze three fundamental network building blocks – an independent design, a radial design, and a meshed design – seeking to develop network design insights, in particular regarding the joint value of postponement and other sources of operational flexibility. The results clearly point to the importance of explicitly incorporating uncertainty, adopting a multistage perspective, and addressing complex interactions between different sources of flexibility, in the design of transmission networks.