Flexibility has become a requirement for modern power systems dominated by renewable generation sources. It can be extracted from different assets, ranging from demand response to fast generating units. This paper proposes an investment model that finds an optimal mix of transmission-level non-generation flexible assets: battery energy storage (BES), thyristor-controlled series compensators (TCSC), and transmission lines. The role of BES is to offset renewable generation in time, but its power converter is additionally utilized to provide voltage regulation by injecting/withdrawing reactive power. TCSC is used to alter power flows and increase existing lines' capacity, while new power lines are used to increase bulk power transfer. The proposed planning model uses a linearized AC OPF and employs Benders' decomposition to develop an iterative procedure for obtaining the optimal solution. The presented case study illustrates usefulness of the model for different BES costs and investment policies.