Hello. When I figured out that no amount of technology could possibly terraform Class-0 planets (at least as of this current patch), I felt peeved by this.
In other sci-fi literature, there are well-known examples of colonies and habitats in what would otherwise appear to be Class-0 planets on GalCiv2.
Bespin, of Star Wars fame, is a Gas Giant. Whereas a Gas Giant is a Class-0 in GalCiv2, Cloud City and other cities on "repulsorlifts" (an anti-gravity device) were possible on Bespin, as well as other gas giant planets found in various places in the Star Wars universe.
Bespin had a habitable band in its atmosphere, yes, so to make terraforming Class-0 planets possible, I propose a 5th type of Starbase- Terraforming. The player would build the starbase in the planet's orbit, and the base would start to slowly make new sectors habitable.
At first, it would take plenty of weeks to turn a single sector green, but when constructor ships arrive to fit upgrades onto this base, the terraforming time would shorten with each new upgrade.
For gas giants with no solid surface- the Terraforming Starbase would be responsible for turning patches of the atmosphere habitable, and improvements would be able to get built in those places. (They're automatically repulsorlifted by this point.) However, a technological prerequisite should be researched first- "Repulsorlifting" or something along those lines.
For hot runaway-greenhouse planets like Venus- atmostrippers could be a terraforming base component to, as said above, strip away (merely thin down) atmospheres to allow the greenhouse gases to pass back into space. Another beneficiary component would be "Orbital Mirrors" to divert the star's heat away from the planet and, as a result, cool it down.
For nearly atmosphere-less planets like Pluto and Mercury- habitation domes for any colonies and improvements there.
For ice-cold planets like Hoth of Star Wars fame- orbital solar magnifiers that magnify the incoming starlight to in turn provide habitable warmth to the planet, which in turn creates green sectors. Another component would thicken the atmosphere in order to better retain the warmth. (Perhaps the name for such a component should be called the "Atmosthickener?" I'm not sure on that one.)
A wildcard of planets is radiation- it could occur more or less anywhere, depending on many things. The terraformer would need a "deradiator"- a component that cleans the radiation levels in the atmosphere or on the surface, and thus make the planet more inhabitable.
For planets with acidic atmospheres and bodies of water- (3 possible names: Terra-Seltzers, Pepterro-Formols, or Alka-Formers) to provide planetary relief from excess ocean acid and atmospheric pain. "Zap, Zap. Splash, Splash. It's all water in a flash!" (Or "Zap, Zap. Splash, Splash. Makes H2O in a flash?")
For pelagic planets (worlds covered mostly or completely by water)- Floating and underwater colonies. Sky cities too, I suppose.
This is all I can think of at this juncture. I invite the devs to share their thoughts.