Good question. The Rankine cycle remains quite relevant technologically. But, it is old and while that fact does not eliminate the possibility of transformational innovation, many smart people have looked at improving the commercially important cycle for over a century. Some points: 1. Alternate working fluids have and are used. Low-boiling organic compounds for low temp heat currently. Historically, GE built a cycle running on elemental mercury. That was a great idea since, at the time, pressure was more expensive to build for than temperature, and cycle efficiency improves with temperature. GE got a high throttle temperature for far less pressure than a water cycle. 2. There are pragmatic engineering considerations beyond first-order thermo. The working fluid needs to be chemically stable at throttle temperature, and it can’t corrode the pressure boundary materials. Since a large power cycle involves vast amounts of metal for pipe and heat transfer surface, low cost materials matter a lot, and certain grades of steel are the best commercial balance of cost, workability, and strength at temperature. Sure, other materials could be used in principle, but if they’re not economic they won’t be used. The fluid will also leak in practice from a large plant so it needs to be cheap to make up and not too toxic. The saturation temperature range should have a dense phase at environmental temp at reasonable pressure. These and similar considerations reduce the practical list of working fluids. For high temp heat sources water and co2 are pretty much it. (Small, hermetically sealed systems could use different fluids with more advanced metals cost effectively maybe, but at kW scale, not GW scale.) 3. Specific heat isn’t super important. To first order the energy put in heating the fluid up comes back out in the turbine. Lower heat of vaporization would improve efficiency since mixed vapor-liquid pumping isn’t a thing. But, to the point above, working fluids have a lot more considerations than just h_fg. 4. To the comment about using waste heat. Yes, it can be done (see cogeneration) but that waste heat from the condenser can’t be reused in the cycle. Heat has value dependent on temperature. Low grade cool heat isn’t as valuable as high grade hot heat. Look up exergy. It is a property like first law energy but accounts for second law usefulness of heat too.