This paper introduces a new co-design approach based on optimization for the modular multilevel converters (MMCs) using wide-bandgap (WBG) switches. This method takes into account several control strategies from the beginning and through the entire procedure. Gallium-Nitride (GaN) mature switches are attractive solutions to increase the compactness and efficiency in embedded applications. It could be a great opportunity for new grid-connected converters, thanks to their improved switching characteristics and lower on-resistance. Nevertheless, their 650 V rating requires the use of multilevel topologies to reach new 1.5 kV DC grids for eco-districts or, more generally, for MVDC grids. The proposed design method is validated using an experimental prototype involving GaN half-bridge submodules (SM-HB).