Radioisotope power systems (RPSs) have transformed our ability to explore the solar system. RPSs have been in existence for almost seven decades. Most missions have utilized 238Pu as the radioisotope of choice to generate electrical power and to produce heat for the operation and thermal management of spacecraft systems. In Europe, for the past decade 241Am has been selected for RPS research programs. This paper hypothesizes that the inclusion of small quantities of relatively short-lived radioisotopes such as 232U and 244Cm, particularly when dealing with long-lived radioisotope 241Am, could have beneficial implications for future RPS designs. This paper focuses on the thermal output implications and impact on system-level design. The authors recognize that the selection of any new or modified radioisotope heat source material will require extensive research on fuel form stability, the radiological impact, cost of production, containment, and launch safety considerations.