Recent studies on periodic metamaterial systems have shown that remarkable properties adaptivity and multifunctionality are often products of exploiting internal, coexisting metastable states. Yet to realize such attractive potential, effecting coexisting metastable states in material systems may require the determination of a periodic constituent which promotes a non-uniqueness when composed within the whole system, thus creating a need for costly, multiscale design. To surmount such concerns, this research first focuses on the development of adaptable, metastable modules: once assembled into modular metastructures, synergistic properties adaptation is found to be a natural byproduct of the strategic module design. Using this approach, it is seen that modularity facilitates a direct pathway to create and effectively exploit metastable states for massive, metastructure properties adaptivity, including a near-continuous variation of mechanical properties or stable topologies and adjustable hysteresis. A model is developed to understand the source of the synergistic characteristics, and theoretical findings are found to be in good agreement with experimental results. Important design-based questions are raised regarding the modular metastructure concept, and a genetic algorithm routine is developed to elucidate the sensitivities of the properties variation with respect to the statistics among assembled module design variables. To obtain target multifunctionality and adaptivity, the routine discovers that particular degrees and types of modular heterogeneity are required. Future realizations of modular metastructures are discussed to illustrate the extensibility of the design concept and broad application base.