While I don't think it's possible to touch 0-level planets, I would LOVE the terraforming tree to be expanded and the terraforming effect to be improved. For the time it takes to research Terraforming, you should at LEAST be able to move class-4 or -5's up to 12's.

Perhaps to terraform a class 0 planet you would need a special ship module, which would cost some completely ludicrous amount, after which you would only end up with a class 1 planet. In addition, the terraforming tech should enable you to slowly improve planet quality of all planets, quicker on the smaller planets, and slower on the larger planets. Perhaps outside the normal construction process, simply give each planet a new terraformable square after X turns, where X could be some small amount (say 10 turns) for the aforementioned class 1 planets, and much much more for the larger planets.

But do keep in mind that the game already randomly does stuff with planets. Once in a while, a class 0 planet suddenly becomes the home of a minor race, and generally the planet is class 15 after that. Also, sometimes planets just become habitable and the UP votes who should get the new planet. Once in a while, it also happens that someone stumbles upon the artifact-of-the-day on a planet, which improves it's quality by 2. And I have had one case of bio-terrorism, another one of those nice random events. My class 12 or so planet was turned into a class 5 planet. Ouch. One of the other races was behind it and it had to be destroyed, otherwise they would continue to do so with other planets.

One thing to consider with this game is the neutral character bonus. Why would you research terraforming when all tiles are useable when you are neutral. That is why I play neutral. However, that being said it is very limited how much you can grow a planet. It means that at the begining of the game you have no choice but to play expansion style builder. If you could build up planets better then you could get a few then just build them. It would allow more options.

I was playing a battle of the gods scenario last week when I first got the game and had an odd encounter with a class 0 planet. The galaxy size was huge and it was down to three species of aliens besides myself and the place was packed with starbases, ships, trade routes and just general clutter, so the game was going a bit slow when zoomed in. Near the end of the game I was clearing out all unneccesary ship maint cost and ran into a colony ship at rest relative to a planet. I clicked on it and went to select "destroy" but somewhere in the process the colony ship was ordered to move to a class 0 planet nearby. I was immediately greeted by the colonize a new planet dialog box. It then took me to a details screen for the planet. It showed terrain with no usable tiles. I opted to "destroy colony", as it had 0 pop and nothing to do there. While being a totally fruitless exercise in resource mismanagment, it does show that class 0 planets can be colonised in the game engine. What a modder can do with that, if anything, I don't know, but there it is...