If your next iteration includes a physical switch to put the fan in 'always on' mode, it will already be superior to the Nest.
Nobody has ever offered this in a thermostat with the possible exception of the RadioThermostat CT30/3M50, which has a JSON API and lets you manually control the relays.
As I understand it any temperature control designed for big refrigerators or freezers has this features. Some even allow you to pre-configure fan cycles (i.e. on for x minutes, off for y, repeat) and turn the cycle on or off depending on temperature or compressor state.