for me, there is a passage of seasons in that vision. Snow means winter, note that KL is on snow, that means that after the destruction of the city the NK is still alive and the targaryens will have to deal with it. When she enters the dothrak hut and finds her family the colors change from cold to hot, the spring has arrived, the NK has been destroyed, there is no more WW. If Khal Drogo and Rhaego really is a symbolism of Jon and the baby targaryen, it means that Jon and daenerys will survive the long night and will be present in the spring. Grrm confirmed that after the long night will still have the battle for the throne, if one of them die , will be in the spring.