The standard approach for treating the electron-phonon interaction is density functional theory (DFT) with so-called semi-local approximations. During the past years it has become clear that this method is insufficient, and can fail dramatically in many systems of fundamental and technological interest such as high-temperature superconductors. To solve the problem, an improved treatment of electronic correlation is needed. Indeed, recent works have shown that more advanced, fully nonlocal approximations in DFT tend to enhance the electron-phonon coupling, and in certain cases substantially increase the critical temperature of materials exhibiting unconventional superconductivity. In this project we will investigate and design nonlocal approximations based on diagrammatic many-body perturbation theory, able to simultaneously capture short and long range effects of the Coulomb interaction. As a first application, we will study TiSe2 an intriguing quasi-two-dimensional layered material where superconductivity competes with a charge density wave instability.