In recent years, substantial efforts have been made, dedicated to identifying lead-free perovskites that retain the remarkable optoelectronic properties of lead-based perovskites while enhancing stability and reducing toxicity. Numerous substitution strategies have been explored, resulting in promising new materials, like for example halide double salts. This class of materials, have demonstrated high potential, with compounds like AgBiI₄ and Ag₃BiI₆ showing some of the highest power conversion efficiencies among lead-free photovoltaic materials[1–4]. Yet, these materials have been a challenge to model due to the existence of vacancies, and partially occupied Ag- and Bi-sites in their crystal lattice. Recently, we developed a symmetry-based approach to create atomistic models that allow the accurately description of their electronic structure and optical properties[5]. Here, we employ this approach to investigate atomic substitutional engineering in order to explore the phase space of Ag-In halide double salts and design a direct band-gap material in analogy with the case of Cs₂AgInCl₆ double perovskite[6]. By substituting Bi³⁺ with In³⁺ in a reduced-symmetry model of AgBiI₄, our first-principles calculations for AgInI₄ show a direct bandgap within the visible range (1.72 eV), close to the optimal range for indoor photovoltaic applications. This value is comparable with the indirect bandgap (1.68 eV) of the Bi-based ternary compound, suggesting a potential advantage for the In-based material in photovoltaic efficiency under specific conditions. To confirm this, we evaluate the spectroscopic maximum limited efficiency of AgInI₄ compound under AM-1.5G solar irradiance and LED irradiance using the LED-B4 standard. Our findings indicate that the In-based compound under-perform with respect to Bi-based for thin-film thickness of around 500 nm, though slightly improved performance is observed at thicknesses of above 1.5 μm. Based on these predictions, we set to synthesize the hypothetical In-based double salt and the resulting thin film exhibited transparency at room temperature, not matching our initial hypothesis. This prompt us to explore the possibility that more stable phases exist within the complete Ag-In-I phase space. By employing a systematic materials screening process within the Materials Project database, [7] we identify new phases that are more stable than the AgInI₄ phase designed starting from AgBiI₄. These phases exhibit direct bandgaps that are larger than 3.0 eV. While these hitherto unknown phases may not be suitable for photovoltaic applications, their transparency, dispersive electronic bands and well-defined band edges position them as promising candidates for other applications that require wide-bandgap semiconductors. Overall, our exploration of the Ag-In-I phase space broadens the opportunities for the development of environmentally friendly materials for optoelectronic applications where wide bandgap semiconductors are required.