ELTs laser guide stars wavefront sensors are planned to have specifically developed sensor chips, which will probably include readout logic and D/A conversion, followed by a powerful FPGA slope computer located very close to it, but not inside for flexibility and simplicity reasons. This paper presents the architecture of an FPGA-based wavefront slope computer, capable of handling the sensor output stream in a massively parallel approach. It will feature the ability of performing dark and flat field correction, the flexibility needed for allocating complex processing schemes, the capability of undertaking all computations expected to be performed at maximum speed, even though they were not strictly related to the calculation of the slopes, and the necessary housekeeping controls to properly command it and evaluate its behaviour. Feasibility using today's technology is evaluated, clearly showing its viability, together with an analysis of the amount of external memory, power consumption and printed circuit board space needed.