Rheological material properties are high-dimensional function-valued quantities, such as frequency-dependent viscoelastic moduli or non-Newtonian shear viscosity. Here we describe a process to model and optimize design targets for such rheological material functions. For linear viscoelastic systems, we demonstrate that one can avoid specific a priori assumptions of spring-dashpot topology by writing governing equations in terms of a time-dependent relaxation modulus function. Our approach embraces rheological design freedom, connecting system-level performance to optimal material functions that transcend specific material classes or structure. This technique is therefore material agnostic, applying to any material class including polymers, colloids, metals, composites, or any rheologically complex material. These early-stage design targets allow for broadly creative ideation of possible material solutions, which can then be used for either material-specific selection or later-stage design of novel materials.