1. Mercury is a crucible map. So it is already in the game. 2. The lore of the game says that we changed and expanded to tons of planets during the golden age. Mars, Venus, and Mercury all would require terraforming for us to not die on immediately. With Venus being just as crazy to live on as mercury. 3. both Mercury and Venus have daytime temperatures around 800 degrees Fahrenheit (400 Celsius) during the day. Venus is actually hotter than mercuy because of it's thick atmosphere(800 for Mercury 864 for Venus). So with the right atmosphere manipulation both would be equally habitable. 4. Destiny ignores science. There are soo many things it doesn't show you about the various planets that it's crazy. For example gravity is about the same on all the planets. In reality mars and the moon have much lower gravity than earth. If destiny was realistic you would be able to jump 3 times as high on that planet and moon.