I always thought it would be fun to have the blight actually work and play through the whole process of turning New Terra into an earth like planet. As colonies would grow, more infrastructure would have to be added, bigger buildings which would eventually look different, because there would be an atmosphere. Tubes would eventually become obsolte, streets would be built, there would be politics, and so on.
Its just the ability to build up a whole new civilization from almost nothing in one game, that I'd like so much. (you don't have to tell me that the civilization series has such a setting. That is not the same, because it doesn't start with the same technological history)