Graphene terahertz (THz) antennas are a subject of intense research due to the tunable properties of graphene that may allow for realizing dynamically steering planar antennas. Still, most of these studies remain at a theoretical level due to the very high ohmic losses exhibited by graphene at THz frequencies. In this work, we propose an original metasurface based on a hybrid metal-graphene unit cell that can profitably be used to realize a high-directivity reconfigurable Fabry-Perot cavity leaky-wave antenna. It is shown that by controlling the graphene filling factor of the unit cell it is possible to achieve a satisfactory tradeoff between the range of reconfigurability and the antenna directivity. Preliminary results are obtained with a rigorous leaky-wave analysis and compared with full-wave simulations.