Metallic magnetic calorimeters (MMCs) combine the excellent energy resolution of cryogenic gamma ray detectors with a very small nonlinearity and a reproducible response, owing to their magnetization-based sensor and their metallic heat flow path. These attributes make MMCs well suited for photon and particle spectroscopy applications requiring the highest accuracy. We are developing high-resolution MMC gamma ray detectors with the goal of improving the quality of key nuclear decay data for nuclear safeguards and fundamental science. Our exploratory "integrated" (SQUIDs and sensors on the same chip) 14-pixel MMC designs recently demonstrated energy resolution of 37.5 eV at 60 keV. Here, we describe the design and optimization for a new generation of MMC detectors using both "integrated" and "split" designs (SQUIDs and sensors on separate chips). The new designs are expected to have an energy resolution < 25 eV (< 5 eV) for MMCs optimized for energies up to 100 keV (10 keV) and have up to 32 pixels.