It would be pretty interesting, in my opinion, if 'terraforming' in the series took on a new meaning beyond "Make moar tiles".
I would like if we were permitted to settle & build upon a 'type' of dead planet; basically ones that are in the star's habitable zone, lukewarm and relatively hospitable (perhaps 10-15% of all dead planets?). From there, we could build "Terraforming Centres" or something along those lines that would introduce life, water and the necessary atmosphere so that the planet could be placed as a numbered class (1-24) instead of "0". The gain from doing this could scale with technology and be retroactive, with early "Age of Expansion" terraforming giving you a class 6-8 planet with only the most useful dead planets being able to be transformed, with the end-game "Age of Ascension" giving you the ability to improve much more hostile environments to 16+ planets.
While I threw this idea together in five seconds, I would personally find this interesting, as it allows you to continually upgrade your infrastructure throughout the game, and gives a sense of realism.