We have used first-principles calculations to investigate the trends in mixing thermodynamics of ScN-based solid solutions in the cubic B1 structure. 13 different Sc_{1−x}M_{x}N (M = Y, La, Ti, Zr, Hf, V, Nb, Ta, Gd, Lu, Al, Ga, In) and three different ScN_{1−x}A_{x} (A = P, As, Sb) solid solutions are investigated and their trends for forming disordered or ordered solid solutions or to phase separate are revealed. The results are used to discuss suitable candidate materials for different strategies to reduce the high thermal conductivity in ScN-based systems, a material having otherwise promising thermoelectric properties for medium and high temperature applications. Our results indicate that at a temperature of T = 800 °C, Sc_{1−x}Y_{x}N; Sc_{1−x}La_{x}N; Sc_{1−x}Gd_{x}N, Sc_{1−x}Ga_{x}N, and Sc_{1−x}In_{x}N; and ScN_{1−x}P_{x}, ScN_{1−x}As_{x}, and ScN_{1−x}Sb_{x} solid solutions have phase separation tendency, and thus, can be used for forming nano-inclusion or superlattices, as they are not intermixing at high temperature. On the other hand, Sc_{1−x}Ti_{x}N, Sc_{1−x}Zr_{x}N, Sc_{1−x}Hf_{x}N, and Sc_{1−x}Lu_{x}N favor disordered solid solutions at T = 800 °C. Thus, the Sc_{1−x}Lu_{x}N system is suggested for a solid solution strategy for phonon scattering as Lu has the same valence as Sc and much larger atomic mass.