Transmission and Distribution system operators are responsible for planning their future network developments. This task becomes more complex with the increased renewable capacity connected to the grid and emerging flexible assets such as storage and demand-side response. This paper explores the critical challenges in grid planning in the context of growing interactions between the grid, generation and demand-side assets. Co-optimizing or co-planning transmission and generation expansion planning helps reflect the tradeoff between generation, load and transmission assets. Because of practical siting constraints of assets, the theoretical efficiency of locational signals does not concretely materialize. Limited coordination between generation and transmission reduces the co-planning efficiency. Moreover, long-term uncertainties, such as the amount of renewable capacity connected to the grid, significantly increase the risks taken when planning grid expansion. Meanwhile, storage or demand-side response provides the system operator with flexible investment options, valuable for investment decisions under uncertaint