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".
I don't want to derail your OP, and I agree with the "habitable zone" limitation, overall, but think it would also be interesting to make some planets, like gas giants, be able to support a colony plus a few tiles. Perhaps these would have to be specialized buildings specifically for gas giants--i.e. a cloud city for colony/capitol & airborne equivalents for manufacturing, research, economy, influence, etc.
This could also be adapted to planets that are too cold, too hot, have no atmosphere, etc. via biodomes and other artificial life support mechanisms. This isn't technically terraforming, but it relates to the use of "dead planets" and those that have a planet class of zero.
Again, I think more options for developing infrastructure is a good thing, but I'd rather the devs focus on the core game first. Terraforming and colonizing other unusable planets could be a DLC or a really cool feature of an expansion.