Running Fire Blast and full SpA investment looks like an OK option. You don't have recoil with that set, or a nature that lowers one of Rapidash' defenses since it is now possible to run an Atk-lowering nature (either Modest or Timid), giving it more bulk and no less offensive prowess as the version previously mentioned. Sunny Day and Morning sun are givens, though, on a special set, making it only somewhat of an option on Sunny Day teams which also need another Sunny Day setter.
But here, we are reaching the point where Charizard is going to outclass Rapidash. Zard also has access to Solar Beam, after all...