We propose a novel multicore fiber design strategy for obtaining a flat in-phase supermode that optimizes utilization of the active medium inversion in the multiple cores. The spatially flat supermode is achieved by engineering the fiber so that the total mutual coupling between neighboring active cores is equal. Different designs suitable for different fabrication processes, such as stack-and-draw and drilling, are proposed. An important improvement over previous methods is the design simplicity and better tolerance to perturbations.